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If you have a few simple changes and you can find the theme designer, try to hire that person - she won't have to waste time getting oriented with the theme and might be able to do everything you need for more per hour, but less overall than a rentacoder.
For something like $250 (check that) you can run a competition on sitepoint for a template design.
A well-known designer who designed the most downloaded theme at wordpress.org had a blog post a year or so ago saying he charged $1600 for a completely new theme from scratch for a major a-list blogger.
Get Firefox if you don't use it already.
- add the colorzilla eyedropper
- the EditCSS plugin
- the Firebug plugin
Using these you can find out where pretty much anything is styled. If you're just changing colors, you should be able to figure this out in an evening.
Otherwise you'll need to get a template, come up with a list of specific changes you want, and then ask for bids. Could be anything from $20 to $500.
It's like asking "How much will it cost to get someone to fix my car"? without saying what's actually wrong with you car.
I tried another 3 column layout that took me a few hours (I'm slow .. not a pro) because I had to make some new background images (designer had used repeated images to look like fancy borders).