Nash, LSL, and K-Line
I wouldn't call these partscasters anymore than I would call Fenders partscasters. They are reputable companies that have been producing high quality S and T style guitars for a long time. They're not selling parts, they're selling full guitars. I don't own any of them but I've tried a few Nash's and I wouldn't hesitate to buy one if I was in the market for a high quality relic (that I didn't want to build myself).
I would never call Nash, LSL or K-Line partscasters. I understand them going for higher prices although to resell at higher prices, you have to target a specific audience who knows about those lines.
As for true partscasters, assembled and setup by someone in their workshop, I wouldn't pay $50 for one. In fact, if I see an ad for a Fender or Gibson that I like and it has any modifications listed, I immediately lose interest.
Bill Nash puts out a quality product. I consider him the reasonable "custom shop" guitars. As far as partscasters go, many sellers try to embellish the description of their guitar when you can plainly see that they are a MIM upgrade. I am not casting judgement on those guitars, but the dishonesty turns me off. Like selling a Squire or Epiphone and calling them Gibson's or Fenders. Give me a break, are you just looking for an ignorant sucker?
The OP is asking about partscasters priced like Nash and LSL not Nash's themselves.
The problem is without a defining brand or model it's hard to track what they actually sell for.