simey - 6:54 am on Sep 11, 2007 (gmt 0)

and not a single paid link I've been involved with has paid nearly enough to compensate me for risking having my site penalised

Well, you're the one who sets the price of the link.

I agree that you have the right to link however you want.
I would say there are only 2 possible benefits of buying a link:

1)increase of traffic
2)increase in SERPS (along with a possible risk to SERPS as well!)

So if one of these is not possible? Maybe explain that to your customers?