Another approach :

NumberFormat format = NumberFormat.getCurrencyInstance();
format.setMaximumFractionDigits(0);
format.setCurrency(Currency.getInstance("EUR"));

format.format(1000000);

This way, it's displaying 1 000 000 € or 1,000,000 €, depending on device currency's display settings

use as follows:

private val currencyFormat = NumberFormat.getCurrencyInstance()

Pass the desired Locale to getCurrencyInstance():

val format = NumberFormat.getCurrencyInstance(Locale("es", "MX"))

"MX $3000.00".takeLastWhile { it != '$' }

is one of many ways.

val result = "[^\\d]+?(\\d+\\.?\\d*)".toRegex().find("MX $3000.00")
result?.groupValues?.last()

works too.

The problem is there's no way to tell whether you started with an Int or a Float. If you started with 3000, it formats as 3000.00 and if you started with 3000.45, it formats as 3000.45. The result will be a Float, and you can choose what to do with it.

This seems to be due to this line:

currencyFormat.positivePrefix = currencyFormat.positivePrefix + " "

Positive prefix at this point depends on the currency of the passed locale. If, for example, you invoke your method as getCurrencyDecimalFormat(Locale.US, Currency.getInstance("EUR")), then your currencyFormat is bound to USD at this point (and currencyFormat.positivePrefix results in $).

Move this line further down, below setting your format symbols. But TBH I'm not even sure why you need it. Having a space after the currency symbol should be locale-dependent and not hardcoded.

fun getCurrencyDecimalFormat(locale: Locale, currency: Currency): DecimalFormat {
    val currencyFormat = NumberFormat.getCurrencyInstance(locale) as DecimalFormat

    currencyFormat.roundingMode = RoundingMode.DOWN

    val symbol = currency.getSymbol(locale)
    val decimalFormatSymbols = currencyFormat.decimalFormatSymbols

    decimalFormatSymbols.currencySymbol = symbol

    currencyFormat.decimalFormatSymbols = decimalFormatSymbols
    currencyFormat.isParseBigDecimal = true
    currencyFormat.positivePrefix = currencyFormat.positivePrefix + " "

    return currencyFormat
}