S.H.
Depending on how good of friends my child is with the child who's birthday it is I spend between $10 to $15 on a present...if it is a very good friend I might spend $20 (this would be for their best friend, usullay I try to stay around $10).
As for the twin situation...one present per child is perfectly accpetable...I would even go as far as trying to find one present both children can enjoy (ie a board game or an outside toy) that both children can play with and get just one present per family (in which case I would spend probably $25). I have a friend with 5 yo twins and this is usually what she tells people to do because she has SO many toys. Like you she has a boy and a girl...so it can be hard, but it is not impossible!
Good Luck!!!