Hi Sweet.Pea 09!
For my treasure box. I let my kids (if they have had a good week--and all usually do) all go to the treasure box on Friday afternoons. My kids come from a lower socio-economic area and a lot of times--the teeny bouncy ball, sticker, pencil, ruler or so on.... means the world to them. I believe in both intrinsic and extrinsic awards--I think a balance is what works for my students. My kids are always encouraged to do what is right--"just because it is"--so I feel that treasure box is okay to do as well.
For treasure box, I try to have parents donate items via the class wishlist--or their unused McDonald's toys. I usually hit up US TOY once in the beginning of the year for a stock up and I am good for the rest of the year with donations.
For my birthday box--(this will be my first year doing this--I'm excited!) I bought different items that aren't in my treasure box--like notepads, little erasers and whistles. Like the PP, I also let them pick out a book. They take home our class Birthday Bag, get to donate a book to our class (and unwrap it at school) and wear a crown (and/or they will get to wear a new headband hat that I just got a the dollar store--it's too cute!)
*For both, I always buy in bulk at a party store, US TOY or Oriental Trading Co so that I get the most value for my $. I usually spend about $30 total for the year, I get lots of help with donations from parents.
I don't make a HUGE difference between the two, the kids are happy to get anything and they are so appreciative--so if the BB and TB have some of the same things--I don't think it matters--well to me it doesn't.

I hope this helped!!

Have fun!!
--Amy