A rough guide I follow.
An empty 19" gutted cab (no wiring, missing coin door, unpopulated CP) = $25
An empty 19" wired cab (JAMMA harness, ISO transformer, PS, CP) = $75
A complete 19" cab (with working halfway decent monitor), no PCB = $125-$150
A complete 19" cab with sharp/bright monitor, generic PCB = $175-$225
If it is a 25" cab and/or a nice big 4 player CP, expect to pay more.