Flat-Tanks and Values

Lenny sent me an interesting question; he's looking for a Sunbeam Model 90 (and who isn't?), and wanted to know details concerning proper engine and frame numbers ('N' and 'E' prefixes, respectively), and where to look. There is a saddle-tank '31 M90 on BoA, (e11,500 - my mac has no 'euro' key), but I mentioned that if he wanted a flat-tank M90, to expect to pay a lot more (e15-20k, or more).