You should use the new format specifications to define how your value should be represented:

>>> from math import pi  # pi ~ 3.141592653589793
>>> '{0:.2f}'.format(pi)
'3.14'

The documentation can be a bit obtuse at times, so I recommend the following, easier readable references:

• the Python String Format Cookbook: shows examples of the new-style .format() string formatting
• pyformat.info: compares the old-style % string formatting with the new-style .format() string formatting

Python 3.6 introduced literal string interpolation (also known as f-strings) so now you can write the above even more succinct as:

>>> f'{pi:.2f}'
'3.14'

Add a decimal point with number of digits .2f see the docs: https://docs.python.org/2/library/string.html#format-specification-mini-language :

In [212]:
"{0:,.2f}".format(2083525.34561)

Out[212]:
'2,083,525.35'

For python 3 you can use f-strings (thanks to @Alex F):

In [2]:
value = 2083525.34561
f"{value:,.2f}"


Out[2]:
'2,083,525.35'

In Python 2.7 and 3.x, you can use the format syntax :,

>>> total_amount = 10000
>>> print("{:,}".format(total_amount))
10,000

>>> print("Total cost is: ${:,.2f}".format(total_amount))
Total cost is: $10,000.00

This is documented in PEP 378 -- Format Specifier for Thousands Separator and has an example in the Official Docs "Using the comma as a thousands separator"

if you just want to format any number into commas for 1000 and 2 decimals, all you need to do in Python is:

"{:,.2f}".format(grossPay[i])