M.C.
Depends on the nature of the friend.
Most of my friends, we wind up buying gifts from an adult store. The last BP I went to was for my sister-in-law to be, and my cousin and I went in together. We got her a few massage oils, a Kama sutra book, and a sexy negligee... Then a few "extra small" condoms, along with some other cheap gag gifts of that ilk... We spent about $50 together.
Although, I have had several occasions where the bachelorette wouldn't appreciate that sort of thing... For those, I usually do self-indulgence gifts... Nice soaps and bubble bath, a paraffin wax kit, gift cards for manicures or facials, etc. (keep it small though... Usually, I don't spend more than $20 for bachelorette gifts.)