actually, purely to "keep cost down" you might want to look at Photoshop Elements.
Generally, anything that works with Photoshop, works with Elements.
Its actually all I've used for years.
Not sure you'll find a side-by-side comparison of what its lacking, but I've added plugins and done many "tutorials" that were for Photoshop, and have never once ended up with "oh, I can't do that because I only have elements"...
I'm sure for professional photographers, its missing all kinds of stuff, but most jobs, its more than adequate.
Only thing I've found missing is the "stroke" feature (hard to put outlines around things) and some of the effects for letters (advanced shadow and bevel effects) are not as full-featured as Photoshop itself.
Anyhow, just an option to consider. Really depends on the kind of work you're doing...