Would you refer a colleague for a job in exchange for a monetary reward?

Nope.

Or rather - If the _only reason_ for doing so was to get some money then absolutely not.

If I refer someone I usually let the recruiter know that I am going to pass on their details to someone I know and leave it to the friend / colleague to decide if they want to apply/approach the recruiter. If there is a commission /finders fee then usually I will ask the recruiter to take me out for lunch and a few drinks. Win-Win on that front because doing it that way helps to keep you at the top of the queue when they (recruiters) think of potential candidates.

The money doesn't play into it for me. I'm not going to sacrifice my integrity or burn a bridge (depending on the context) over a few bucks.