I don't have a 'budget' so much as a list of forthcoming releases that I know I'm going to purchase, which I try my best not to deviate from on impulse.
For example right now it extends to
- WFCS Apeface
- The remaining Studio Series Constructicons (I have the first four already)
- Studio Series Shatter (jet mode)
- ER Wheeljack
- ER Starscream
I find that focusing on that keeps unnecessary spending down
(though I know I may be tempted on a whim with many of the coming Studio Series deluxes like the Arcee triplets and Soundwave).