I didn't buy something for my secret santa giftee.
That's not good. Do you expect to get anything in return from your secret santa giftee?
I only wanted to give them a gift for a few reasons, but I don't know what to gift them.
If you signed up for something like a secret santa then you should give the person a gift. It's considered very rude and tacky to not buy presents when you say that you will and the others involved may kick you out from any sort of fun events after this one. You should go out and buy them a present even if you're not sure what to get.
I'm bad with spending money. I just made the budget for it and 100 is too much for a gift.
Well you don't have to spend all that on the gift, you just have to be fair and get them something of equal value. Otherwise you will not be invited to participate again next year.