I have done several of my own. I can't tell you what it costs to hire someone but I can tell you how much time it takes for various repaints (below). If you know about what people charge per hour for repaints you can get some idea of what it would cost. I do know from talking to others that often the repaint costs as much as the loco did. One person I knew who did it (no longer does) charged about $25/hour for repaints.
The actual repainting itself is a small part of the time required on many locos. If stripping the old paint, sanding, smoothing old corrosion, filling nicks and cracks, etc., gets very involved, that can take a lot of time: I spent over 24 hours just striping paint, sanding and filling pitted metal smooth, and priming a beat-up postwar steamer. Multiple colors - a complicated livery - can eat up time. You have to do the colors one at a time and mask the boundaries between each: getting the edges correct and crisp between them, can take a lot of time.
Easiest is repainting a new or nearly new steam loco black. Both locos below were repainted when new. The Pacific in the foreground was a Legacy Southern Crescent - green and red, etc: about eight hours was all that was required to mask, prime and repaint both loco and tender. That behind is a Legacy Berkshire that was black, but I put a new cab on it and made some other changes and had to repaint the black, including the tender. About ten hours - the masking around the silver made it more work.
The little pre-war Marx loco below took about twenty four hours over several days (for paint remover to work, for filler to harden, primer to dry, etc): had to strip paint, do "body work" on dents and scratches, fill, smooth, prime, then paint. Maybe twenty hours.
Different colors side by side require both painting the loco several times (once for each color) and careful masking to get the lines between colors correct and crisp. The loco below is a Legacy N&W J repainted UP greyhound. About twenty hours total over a period of five days (they black is the original paint, I did the gray next and had to let the gray dry for four days before masking and doing the yellow, etc.)
The Trainmaster below was once some other railroad than UP - I forget which. This took about twenty hours including decals, over two weeks (did the yellow first, then the gray, then the red).
I have not tackled multiple colors with curved boundaries - that would be more challenging yet. Something like ATSF's Warbonnet would be very difficult to do. I plan to do one someday, on something cheap, just to learn how to do it.