I had to have a vet euthanize a gecko once. It was actually an injection into the main body cavity (sorry, I don't recall what substance was used), and in the case of an underweight, severely sick leopard gecko, at least, it worked almost instantaneously. If there was any pain, it didn't show; there was no reaction except that he had obviously changed states from alive to dead. I was emotionally messed up about it for quite a while, but I can say with certainty that my little guy didn't suffer in that moment. In point of fact, his suffering was over; his health had been declining for some time, and the prescribed treatments weren't bringing him back to health.

IMO, long term, at least, you won't be worried about the $50; you'll feel better that you gave your scaly friend an easier end.