Eh, just an idea:
full or half plate mail, 10% weight increase, masterwork, the shell is worth 500gp and they can subtract that from the price if they're crafting it themself (that sounds funny, pardon,) and it has armor spikes built onto it for free.
You could describe the how the pincers now stick straight up out of the shoulders, etc.
I'd only make it half plate if they don't have anything better by now.
You could also let cost less to do ankheg specific enchantments on the plate -- although I don't know what the heck those would be.