I've had many small pets (mice, guinea pigs) and had to make decisions like this several times. It's very hard, I know. The fact that is "just" a hamster doesn't matter- if I care about a creature, what it is doesn't make a difference when it comes to how hard it is. However, it is not realistic to expect that every time a small pet has a problem you can spend a lot of money on operations. Sometimes it's worth it, other times it's better not to. In some cases I think it's kinder not to put the creature under the stress of going to the vet and having an operation. If she isn't in pain, it might be better for her if you pamper her at home so she continues to have an enjoyable life, and then, if she seems to be in pain or uncomfortable, have her put to sleep. But do you know the exact price and the chances of a full recovery? Calling the vet and asking might not cost anything, and the vet will know best if you ought to have the tumor removed or not. Whatever you do, don't beat yourself up about it. You're thinking about her best, which is more than most people do when it comes to a hamster.