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There is an overall lack of thread because carpentery now requires it as well as tailoring. So basically it is in extreme demand, a vendor who sells this for 4-5 copper each whould be reasonable I think. The price is high first of all because of high demand and low supply, and because it decreases player interaction, but also so you can't profit unless you sell to other players balancing it out. Since vendors have limited supply of it I don't think its a problem. You basically have to be a warrior to get thread, which means your going to reach your skill cap fighting like 8 mummies by the time you get like 4-5 thread because craftsman are not fighters with armor and will gain experience in several skills because of the amount of health they loose. You might say "find a warrior who will get them for you". Simply not profitable enough, you can't sell anything from mummies to vendors, so most warriors do not fight them except for a helmet or extra weapons. For leatherworkers (rp wise, since cloth and leather are governed by tailoring) they have to kill pigs or buy leather ontop of it, so they might as well consider being a mercenary by the time they can make anything more than shoes.
Pigs and sheep have suitable insides for the job, I think this is a good option too.

Or having all mummies carry insides, not just the uber ones with helmets

We are just fixing that.

Oh man, Im good and you are to Gro'bul but not as much