If your not comfortable with an angle grinder... dont do it!
I like the idea about asking your local chapter for help.
As for a price, it is very hard to say, and allmost definately gonna be considered 'too much'. Most of us are cheap, and shop labor aint cheap.
If you deceide to give it a shot, and wanted practice, you could allways mark it just barely above the existing arch, and give it a shot. Use good cut-off wheels, (not harbor freight), and make sure they are in good condition, without any cracks/fractures. Use a full face shield, and be carefull. Since you are cutting on an arch, its not just 'plunge-cut, and let it run', Think of it more as a small step past a score'd cut. Once you get the hang of it, its really not difficult to cut on that arch. Just remember which side of the cut you want to keep, you can leave extra, and smooth it out with a flap wheel, but if you cut too much, its harder to fix.
