So, I just found out that there is an extra $300 in the budget for me to buy stuff for my math class. What are some recommendations of good stuff you guys have bought/used?

I'm SO not the right person to be answering this question!!! I would spend a little on colored chalk and give the rest back.

For $300, your best bet might be a good set of manipulatives. Even HS students can still benefit from actually "seeing" how the math works.

Algeblocks...if you don't already have them or other manipulatives. Or a good software program to make 3D graphs, trig functions, and other areas of math come alive.

I second the manipulatives. I teach Grade 9, so I can only speak to that, but we use algebra tiles for our polynomial unit and geoboards for our circle geometry unit as well as our similar triangles and similar polygons unit.

Versatiles were a great hit in my classroom. Students can work independently and check their own answers by matching the figure. There are versatiles for all levels of math. http://www.etacuisenaire.com/versatiles/versatiles_math.jsp#think