50% does sound like a big amount, take caution with ICOs that offer that much.
15%-20% is a more realistic offer.
If the amount sounds ridiculous from your perspective you can always talk to the dev team about it.
Well, I think 50% is still can be fine...I would be much more suspicious about projects which offer 100% bonus straight ahead ...That I don't understand indeed - why they may dissolve the value so much offering tokens/coins for free???