1) It's what the market will bear. Historically, instruments with GPIB have been the rather premium gear. People who can afford that class of gear wouldn't blink at only $150 for an interface.
Of course, now that all that old GPIB gear is being replaced with newer, shinier (faster/cheaper/better/more features) gear, the landscape is different.
2) The interface itself (hardware) is probably not a major part of the cost. You could do it with almost any $15 Arduino-type board.
The big cost is probably the software drivers, which includes LIFECYCLE SUPPORT through all the revisions that the OS people (MS, Apple, et.al.) throw at you.
At least that is my perception of the issue.