The bottom line [at least for British guns]
Shallow frame: [ Boss, Woodward, and Purdey] Retail at over 50K

Deep frame: H&H, Lang, Lancaster, Westley, Green,Baker, etc. You are lucky if anyone even looks your way; generally less than 20K or even 10. Early Purdey O/Us[pre- Woodward patent] also go begging.

I guess its about asthetics not quality; A very fine BEST O/U can be had for a considerable discount even compared to a SXS if it has a "deep" frame [traditional hinge pin and locking lugs].

All the best, Mal