Not my expertise, but if I had to guess, the most inexpensive would just be a paper or plastic decoder... I mean, they give those out for free, don't they? I guess you'll need one for each brand, though.
I think they just come with most brand-specific pin kit, kb9. If you buy them separately I can't imagine it costing more than a few bucks; it's basically just a card with a "stairstep" cutout in it. You put the key in to the depth of the cut in question, then slide it sideways until it stops. The number above that point is the depth.
