I must say, it really depends on two things:
1 (Most important) - How well do you enjoy the server? What type of server is it?
2 - What are the offers?
If it's some DarkRP server and it's $15 for just "cul nocleep", not worth it
If it's HL2RP and it's offering operator or flags, then yeah it would be worth it, but the main one of all is;
How well do you enjoy the server? Do you really want to donate to it?
I once donated $60 to a HL2RP server and got OP, as well as infinite custom models from TnB, plus flags.