Foes has a replacement hub with disc kit that'll bolt onto any stock style fork arrangement, or you can send your leftt fork leg to BBR and they'll modify it for their (Polini) disc/hub set up. Or if you can weld or know someone, you can do pretty much anything you want. Buy KTM or other caliper, master cylinder, etc. to complete the brake system, plus a hub or complete wheel, then machine a bracket and weld it on. What I'd like to see is for Reeg to make a front kit to match their new rear foot activated disc brake kit. It's to simple that I'm surprised no one else has done it, but it sure is slick. They sell as soon as I get them in and I think a front kit would be even more popular. (Are you listening Reeg?? heh)
How much will "what" run for? The Foes set up? Reeg's rear disc? I mentioned a few different things in there and don't know what you're asking about?!? Send me an e-mail or something and I can pass that info along.