What to give as a gift at a wedding?

I know we are giving them money my sister in law and her husband but I don't know how much to gift. Shes made comments about it costing around 50 a plate and that she thought the rule was u gift cost plus 50. Ive heard of covering you plate but not plus fifty. I'm also a little annoyed as at our wedding she gave us a 200 dollar wii (which was an awesome gift that we really appreciate) but based on her thinking her entire family cost us 450 (just the reception not even including there accomadations and attire we paid) to have at the wedding plus her 50 rule she should have given us 500 (which I would hope most ppl think is ourtragous cause i do). She didnt even invite our kids but thats not really an issue I just dont feel comfortable giving this women 150 bucks. Is 75- 100 acceptable?