Common is cheaper....any tartan that has to be custom woven is not from the "common" masses. If you can go to the back of the mill and get the roll from the shelf it is going to be a Hell of a lot cheaper than "weave this for me"
As for kilt makers, there are the "traditional" hand sewn or the time saving with visual shortcuts of machine sewn. Each make beautiful kilts. It depends on what you want and expect. If I were ordering a custom woven tartan, I would want it made so it can be altered down the road to remain a family heirloom. Make sure which ever kilt maker you pick (and they should be able to help with the custom woven process and price)they add facings etc. for future alterations.