I always try to keep an equal amount of presents (since we alternate opening gifts), but the amounts are always somewhat off.
I think it would be too hard to try to keep the amounts exactly the same.. but in my case, I do try to at least keep them similar in price.
For example, this year for their main gifts, my younger dd is getting an American Girl Doll ($100).. older dd is getting an Itouch ($200).
So for younger dd, what I'm doing is buying her 3 outfits for the doll as well as the book set (approx. $25 each).
Then for older dd, she'll be getting 4 smaller accessories for her Itouch (case, screen protector, 3D player, and $15 gift card). So it'll be the same amount of gifts, just different cash amounts.
Of course, they're then getting smaller gifts that will be about equal in price and number.
We also always do some combo gifts that go to both of them from Santa... such as this year, it'll be a new bounce surface and springs cover for their trampoline, a small box of webkinz clothes that they'll share, and an air hockey table that I found on the side of road,

, and bought new fan for it for under $15.