I have owned and played Halo 1, 2 and 3.  I never owned ODST, but I've been feeling compelled to pick it up for the multiplayer, but I try to limit how much I spend on games so basically my question is, is it worth it to pick up ODST when Reach is so close, or would it kind of be a waste of money.

While ODST is a great game with a fun story, some exploring, firefight and the entire Halo 3 multi exra, I don't think it's worth it with Reach around the corner.
Reach will also offer Firefight (the one thing ODST had over Halo 3) but with matchmaking and way more features overall.
 
Safe bet that most of the Halo 3 players will jump to Reach too.

 Maybe to familiarize yourself with the rules of firefight? But with Reach out in about a month I don't think you're missing much.  

It depends a bit on how much you like Halo but it should be noted that ODST was never created to be a large single player experience. It has a short campaign and the firefight mode in there and if you're really into the Halo 3 multiplayer it has a disc with that on it along with all the map packs, however considering that Halo: Reach is going to have its own multiplayer you may find this content somewhat redundant in the near future.