Used to happen quite often in the early '90s LHR-MAD, there was a large group of us working out there for months on end, but coming back every other weekend. Seemed like evey other flight they were offering a couple of hundred pounds for volunteers - like others above, we couldn't take it as we had to be there.
These days, it's our company's policy that we're not allowed to accept such offers 'for personal gain'. Mind you, the same is true of gifts - didn't stop me accepting the 3-foot long jade boat I got from a supplier as my leaving present from my time in China!