I just saw a widebody head cab on the bay close for a bit over $100 ...cheap IMHO prob because the wides aren't as popular. Short chassis Marks are regularly in the $150-250 range used, depending on the condition. For reference, I just bought a superb Mark IV headcab for my MKIII from a kind gent in TGP forum ...$170 in mint condition!! If you're patient and can afford to wait, they do come up now and again.
Sidenote to any MKIII users wanting to convert to a headcab ...buy a MKIV cab! Much better access to preamp tubes!!
Edward