Learn English – What’s the opposite for “steep learning curve”

I'd say shallow or gentle are suitable...

So, to be clear, you're looking for a way to say "for money" in Latin: using a "pro" construct that's not in the list of phrases that are commonly used in English.

Perhaps "pro denario" (singular) or "pro denariis" (plural) -- where 'denarius' is Roman currency -- or "pro argento" (meaning silver), or "pro pecunia" (money), or "pro salario" or "pro honorario".

If you want to say that you're doing it in exchange for something (usually money), you can say that you expect a "quid pro quo", which is a well-known Latin expression.