Spend as a noun does not yet seem to enjoy general mainstream use. Merriam-Webster's Eleventh Collegiate Dictionary (2003) has no entry for spend as a noun—and it appears that Merriam-Webster Online has yet to add an entry for it there. The American Heritage Dictionary of the English Language didn't offer any information on spend as a noun either, as recently as the fourth edition (2000). But the fifth edition (2011) of AHDEL provides this coverage:

spend ... n. 1. an amount of money spent on something: doubled the spend on computers. 2. The spending of money; expenditure: the management of spend.

From the examples given, it seems clear that both spend (1) and spend (2) could be replaced by spending without changing the underlying meaning in either case. But if spend isn't different in meaning from spending in either of the two senses that AHDEL identifies it as having, why does it exist at all? When a word catches on in some precinct of the business or academic world despite not enabling people to make a fine distinction between one thing (called, say, spend) and another (called, say, spending), the logical explanation is that the new term appeals to them for some reason other than its contribution to greater coherence—its value as a marker of up-to-date jargon fluency, perhaps, or its perceived jauntiness.

In any event, spend as a noun in the two senses spelled out by AHDEL has not yet gained the same level of acceptance as spending, as is evident from this entry in the in-house word list at a business consultancy where I do a lot of freelance editing:

• spending: consumer spending, marketing spending (not “spend”)

I have no doubt that spend as a noun is well entrenched in some areas of the English-speaking world, but it would be a mistake to suppose that it has achieved parity with spending everywhere. A comparable though even less widely recognized term that has emerged from the land of MBAs is ask as a noun, which carries the meaning "question, request, or inquiry." (AHDEL doesn't have an entry for ask as a noun yet.)

Part of word choice is a simple matter of knowing what words are possible—that is, what words are in use (or at least usable), and therefore understandable, in certain English-speaking populations. But another part of word choice involves having a sense of the tenor, reputation, or prestige that a word may have among readers or listeners. It is surely worth noting that spend as a noun has far less currency than spending in the English-speaking world at large.

Update (April 29, 2022)

In a comment below this answer, site participant DjinTonic points out that an entry for spend as a noun appears in a 1991 edition of Collins English Dictionary—much earlier than I would have expected. This led me to check some late-twentieth-century dictionaries that I have on hand to see whether spend as a noun may have originated in the UK and only later migrated to the United States.

Neither The Concise Oxford Dictionary, sixth edition (1976) nor Longman Dictionary of Contemporary English, new edition (1987)—two British dictionaries—has an entry for spend as a noun. Similarly, Merriam-Webster's Eleventh Collegiate Dictionary (2003) and The American Heritage Dictionary of the English Language, fourth edition (2000)—two U.S. dictionaries—do not acknowledge spend as a noun. But a couple of U.S. dictionaries from roughly the same period do mention it.

Here is the relevant entry from The New Oxford American Dictionary (2001):

spend ... n. informal an amount of money paid for a particular purpose or over a particular period of time: the average spend at the cafe is about $10 a head.

And here is the one from [Merriam-]Webster's Third New International Dictionary (1986):

spend n the act or process of spending money — used in the phrase on the spend

The Oxford Universal Dictionary on Historical Principles, third edition, revised (1955) has a similar entry:

Spend, sb. 1688 {f[rom] SPEND v.} The action of spending money. Only in phr[ase] on or upon the spend.

The 1688 instance that the OED cites is probably from John Bunyan, The Advocateship of Jesus Christ Clearly Explained, and Largely Improved, for the Benefit of All Believers (1688):

Now, set the Case again, that some ill conditioned Man should take Notice, that these poor Men live all upon the spend, (and Saints do so) and should come to the good Man's House, and complain to him of the spending of his Sons, and that while their elder Brother stands by: What do you think the elder Brother would reply, if he was as good-natured as Christ? Why he would say, I have yet with my Father in store for my Brethren: Wherefore then seekest thou to stop his hand?

In fact, published instances of the phrase "upon the spend" (or more precisely, "live upon the spend") go back more than 350 years. From Thomas Doolittle, A Spiritual Antidote against Sinful Contagion (1667):

We too often lose some degrees of our Grace, of love, and Faith, and Hope, and too seldom complain thereof to God. But though we do not so often decay in temporals, we too much complain to man, and murmur and repine against God. Do not many finde some inward frettings in their hearts, that they live upon the spend, and nothing coming in; so many to maintain, and their shops shut up! What trouble is it unto some that the other day did live in good repute, and were esteemed to be rich, that now must be constrained to borrow or to beg; that the other day they hoped should be rich, but now are effectually convinced they are poor; that if they escape the stroke of Death, see themselves falling into the depths of poverty and want!

And from Mrs. Christian Davies, The Life and Adventures of Mrs. Christian Davies, Commonly Call'd Mother Ross (1743):

Our Regiment was quartered in Ghent, where I was delivered of a Child before my Time, which lived about Half a Year. Rather than live upon the Spend, an idle Life, I hired my self to Mr. Dupper, who, since, kept Tavern on Fish Street Hill, and was then head Sutler to be under the Cook.

Although the idiomatic phrase "on the spend" is uncommon after the eighteenth century, it appears at least as late as Thomas Cooke, The Universal Letter Writer: Or, New Art of Polite Correspondence (1841):

Those men whom business does not call out to get money , are generally on the spend ; and he that is driven from home by a wife's ill-humour, is always more extravagant abroad, and even thinks he has a better pretence to be so, while he sacrifices his body and soul, as well as his estate, to his revenge.

I doubt that the old idiom "on the spend" had any significant influence on the emergence of spend as a noun in modern business jargon, but it does provide spend as a noun with an impressively long pedigree in English usage.