My husband's niece is getting married in September. (Yes, technically now that I'm married to him, she's my niece too, but she was 18 when I married him so it's hard for me to get used to.) I've spent about $100 on a shower gift and plan on giving $100 as a wedding gift. I think that's reasonable for us.
Honestly, she is the first to be getting married, but she's also the one who treats us the best. Her father is my husband's half-brother, and they didn't exactly grow up together. And we live in MA and they are in NJ, so we're not close. But when we're with them, we are treated very well. Hubby's other half-siblings only treat us well when they want something and some of their kids, I won't be doing anything for. The niece who is getting married made a point of telling my MIL that she really wanted hubby and I at her graduation party - so we made sure we went. And before the date was even set, she made sure to tell me that she really hoped we could get there for the wedding. So we do what we can.
Now my niece and nephew, to be honest, I'll probably do even more for them. But I lived with them when they were babies and I've had a hand in raising them. We are extremely close. I'm also my niece's godmother. And I forget what it's called, but my sister and BIL have made me financially responsible for the kids if something were to happen to them. As BIL is really their step-dad, obviously biological dad would get physical custody, but I would control their college funds, etc.