Yes, I agree, the adaptors are quite expensive compared to the price of nearly any used PowerBook. In comparison to the price once to be paid for a new 2,5" SCSI harddisk it seems pretty affordable, like a fifths of the price for ten times the storage capacity with even improved performance. A do-it-yourself solution like the homebrewn adaptor Bunsen suggested is more suitable for a time millionaire. Usually it is difficult to build such thing cheaper from scratch than to buy the same thing from an industrial supplier (as long as you do not take the fun factor into account, as a bonus
The high shipping rate is another problem one could deal with. How about bundling the overseas shipping to a location in your own country and distributing single adaptors to their final destination in the same country for the cost of a local ground service? At least USD 10 should easily be saved per unit. Probably the original supplier will give some discount in case of a bigger order. Do not underestimate the hassle of picking up things at the custom office, collecting the money and shipping things to customers. This should be organised professionally, preferably by someone already running a business. In the end the price cut might be neglegible, however.