You could do a custom upper that you'd have to make yourself with a buried block if you went with a comp length upper, standard gas system and a 12" bbl....but you're going to have to cut the excess length on the comp upper, then have it re-anodized or cerakoted. It would look cool (almost did this on a 6.5G), but it would be a lot of added expense and work and you could fuck it up.
As for cost....go to Rob Arm's website for the MSRP. Contact Navalbeaver or Gramps96Kz here for pricing as well. I think factory pricing for the standard length uppers are in the $2100 range currently with a lengthy wait time of at least 12 weeks.