Sponsored backinks

I've got a reputable News site which does very well on Google Search. I've always ignored the incessant requests for links to be placed within articles - especially as many come from dodgy Gmail addresses with broken English. However, it's hard enough to earn money in journalism and if these guys actually pay, then they're better than many legit industry players. So, is adding these links harmful? Do they actually pay? How long do they expect a link to be there for (i know some publishers only let them stay up for 3 to 6 months)? What's the going rate? I see plenty of tales of people making a fortune from these things but the ones I've worked with don't seem to have much to spend.