Here is what we do in my family.
On Christmas eve, my sister and brother (who are both 20+yrs older than me) gather at mine and moms house. My sister brings her two sons (26 and 30) and daughter (16 and 14) and fiance. The two sons also bring their significant others. My brother brings his wife and son (2yr old) and his daughter (20), usually shows up on her on (or with a bf, usually without letting us know
).
Mom buys for everyone, but the gfs and bfs unless the bf and gfs have been around a lot and we know them. This year we havent' met Dans gf, so we don't have anything for her. And we didn't buy for the boy the 20 yr is living with/dating (mainly b/c she doesn't ask if she can bring him). But everyone else has a present (in the $30 range) from mom and I.
My sister and brother only buy for mom (she gets LOADS of stuff from them, plus cash), and the kids under 18. So mainly everyone has one gift, the kids under 18 have 2 or 3 and mom has bunches.
I think its perfectly fine for you to spend more on your mom, after all..she is your mom. Think of how much she has done for you!