I recently printed my star map at a local photo shop (not copyshop, but I guess those probably offer the same services). A1 size cost me 20 Euro (guessing about 28 US$), I think A0 was 30 Euro but don't hold me to that. Do a smaller scale test print first; I got something the size of roughly A3 for 4€ done as a test. I recommend using matte photo paper, not glossy. They should be able to handle just about any bitmapped file format - mine was perfectly happy to eat PNG but since I had both PNG and JPEG the staff picked the JPEG. (For that format, ensure to save at zero-compression JPEG, of course.) It's very important that your graphic has a high-enough resolution, or it will just end up looking crappy. I am not an expert but mine is 200dpi and it looks great; I am guessing 150 dpi is the minimum you should go for.

As for comission rates, post in the mapping request forum, or find guild members whose style you like, and ask them directly if they do comissions and at what rates. Last but not lest I think there was a discussion on comission rates in a general forum somewhere some months ago, you could search for that.

Last but niot least, software... eh. Personal choice, I'd say! Photoshop, Illustrater etc are all very expensive unless you can buy a student version, so free alternatives are probably what you want to look into. However, as long as your software can export to a bitmap format, you are good to go. Personally, I do most of my work in Inkscape, but I've recently started to add GIMP to the mix. As with so many things in life, there's no one right answer; the choice of tools depends on what exactly you are trying to do.

Hope this helps.