In my experience how much you pay seems to relate to how much you want it multiplied by the closeness to payday..
Only joking
Let's say a starting price for a Solid, average Yo frame only is £370
Let's think about the factors
Age - Mass bikes tend to be more sought after than NY bikes. Original Fat, originsl welders etc. NY made in the Serotta 'shop. NY ones in my opinion are better made (i know its not nice to say) and on occasions use older tubing so you get full bullet stays from the old machining.
NY -£30
Mass +30
Rust
If the seat tube is holed or has been replaced - £70-130
Paint
Needs a respray -£90
Patina showing a bit of history -£30
immaculate +£50
Unique + £120
Many paints but some seem to garner more interest than others.
Harlequin ££££
Aquafade, Milka £££
Metalica fade, Sapphire Blue, Cherry ££
Solids £