How much should I pay for a second-hand Marshall amp?

Ok, the problem is I don’t have the exact details, I don’t know which model it is and so on. It looks medium-sized, I know it would probably cost more than $500 new, and I tried it recently and it sounds good. There’s a problem with one of the buttons and when I took it to be fixed the guy at the shop said it would cost 95 euros (a bit over $100) to fix it. My friend said he can’t afford to pay for it, so he said that if I want I can have it fixed and keep it. But I know it’s worth a lot more than $100, even in this condition, and I’d like to also pay him a bit more.