For a monetary gift, simply thank the person(s) for their kind gift - never mention the amount. It is nice if you can indicate what you might use the money for, such as key money for an apartment or your honeymoon.
Tell them Thank you for the gift of money. If you choose to you can tell them what you are going to use it for. For example if you're going to save it to buy a house or something you can tell them that.