I would concur with Billfred, T-shirts are going to be the most important investment in your PR efforts. Several years ago, when we were on a tight budget, we made a deal with a local pizza place to buy pizza's at 5 dollars per large, one topping pizza. We sold those to the students for $1.50 per slice. The profit went into our t-shirt fund, and allowed us to buy shirts for every student. A distinct look will go a long way towards creating a recognizable image, and allow other teams to remember who you are.
Secondly, with the balance of the funds, you have other options than just buttons. As mentioned above, stickers are always a good choice. You can also create small items, that are easily customizable. For example, we have given out glow-in-the-dark bracelets, and added a small tag with our team number on it. I have also seen teams that have gone to sites such as
www.orientaltrading.com, bought items in bulk, and then wrote on them with sharpies. You could always get something that is either based on your team mascot/image or based on a game element. One year a team bought flashing soccer balls and wrote their team number on it. I still have one on my cork board in my office.
Lastly, don't overlook going to local businesses and sponsors and see what kind of swag they may give you. Many medium - large companies will have boxes of swag that they use for marketing. They may be willing to give you a box or two to give out.
Be creative and have fun with it!
Steve