I ordered my custom Mark IV headshell direct from Mesa, and have also odered custom headshells for my DC-2 and Mark I. Here is the cost breakdown:
Base price w/black taurus tolex (empty shell - no reverb) - $249
Reverb tank installation - $59 (I highly recommend if you can afford it)
Standard upcharges for tolex and custom finishes apply. For example, if you order yours with custom tolex it will cost you $179, which includes your choice of tolex and grille (black, gold, tan). My Mark IV headshell cost me $487 before tax and shipping (orange bronco tolex, tan grille, vintage Mark I "Boogie" logo, reverb tank installed, tan leather handle and corners). This should give you a very good idea, the only variable will of course be shipping.