depends which type but since this is in the mk3 section i guess you mean an AQX/AYP lump
in which case i'd be looking to spend between £500-£800
however tbh if i was going to buy one personally i'd either pick up a latter Golf lump AUM code i think or a BAM LCR lump and just remap it
reasons being...
if I was going BT i'd go for the golf lump since the stock turbo isn't relevant it doesn't matter what it comes with (k03s). prob around the same price as above 500-800
but if i was just fitting a lump and remapping for the standard turbo, might as work with the k04. looking at over a grand probably