I came close to accepting the same type of compensation (it was $300), but one person in first didn't show, so the seat was able to remain empty without moving anyone. Unless it was a 5 hour flight or more, I'd volunteer again without hesitation if I was flying on an upgrade (in my case, it would be my segment upgrade on DL). I fly 70% on my own money and $300 can pay for almost an entire upgradeable roundtrip on certain itinieraries for me.