Most frequently used tools & what I paid:
Allen wrenches - $1 for a set
Screwdriver - $1
Set of feeler gauges from an auto parts store - $4 or $5
Capo - $15 or so (but it's a good one that I use on a g#%tar for actual playing, cheaper ones oughta do well enough)
This is a bit "unconventional", but I have a rather finicky MIM Fender Jazz. I found that adjusting the relief by using the low-E as a "straight-edge" was a bit clumsy. Still, I didn't want to spend $50-75 for a "precision straight-edge".
For me, I found getting a $12 aluminum drywall square, and cutting-off a 24" length of straight-edge was "just the ticket". Naturally, after cutting, you'll need to file-down the rough-edges of the cut so you don't scratch your bass...
To the list above, I'd also add a 6" ruler that measures in 64ths. You should be able to find one at Radio Shack or Home Depot / Lowes. The feeler gauges are good for relief, but I also like something to precisely measure string height.
Note: Once I got the feel for my bass, and optimized the relief & string-height combination to the limits of my fretwork, I don't bother measuring relief. I just "eye-ball" the relief and confirm what I think I see by where buzzing is: low-frets only or high frets only.

