Is photoshop the only program that you can use to make custom textures? Are there any others that aren't so spendy?

I believe The Gimp can do pretty much everything needed to create custom Doom 3 textures, although it might take some time to get used to if you are a Photoshop user (something which Gimpshop tried to fix).

Any program capable of creating TGA JPG or DDS image-files can create Doom3 textures.