I use single stage and base/clear quite often. On things like my frame, axles, misc. brakets and other parts, I use ppg's DP/LF series of epoxy primer with ppg's MTK line of single stage urethane paint.
On the vehicle itself I use ppg's MTK line of base color and ppg's Concept DCU 2021 clear. I think your going to get a deeper / higher gloss with base / clear, but single stage is also nice. You probably could save a few bucks by going with the single stage, but not to much. I don't what the body shops are quoting you, but the material difference in paint wouldn't be to much, probably a couple hundred bucks (if that) more for base / clear. The prep work to get to painting is the same either way, also time to spray is about the same. Depending on who your getting paint through, a gallon of MTK single stage ureathane is about $100 bucks, $40 for the catalyst, $25 for the reducer. MTK's base is about $80 a gallon, $220 for a gallon of Concept DCU 2021, $40 for the catalyst, $25 for the reducer.
I don't know if your doing inside and out and the bottom, or just the outside? If you were doing inside, out and bottom you probably would go through a couple gallons of single stage. You would also go through a couple gallons of base, but you would only need a gallon of clear. So really there isn't a whole lot of price difference.
I have some pics of my cruiser project on my website:
http://www.crustycruisers.com/mattpicpage9.htm
Pretty much the only color on my cruiser that is base / clear is the yellow. My white hard top is single stage, along with frame, axles, misc. brackets, all interior metal components, etc....
Both paints spray different, but as far as ease or difficulty of spray, both spray about the same that way.
Hope this helps, Matt