Peter,
I have found that the dollar difference is marginal, depending of course on which paint you use, but generally just a tad cheaper to use Auto paint.
The big difference I find is in quality of finish, higher shine etc and is far more durable at least for our boats, and I have now either done or had done several boats, from Moths, thro' A's and our mossies(plural).
After all, auto finishes are designed to get wet, be left in the sun, range in temps from freezing to our Aussie sun (40+ C) etc etc.
Youngest's boat which took out "best presented" at last nationals was painted in auto finish but my mate....and after a hard season racing still looks the part.

Happy to give you paint details if you ever want them.

Ciao, David