There's a lot of things involved that will affect price. For me it was that I tore into it, prepped it, did the fatmat, did the carpentry and had already bought all the parts I wanted installed. (it took several years btw) It was clear what I wanted, and this helped so the shop could focus on the details.
Even so, my interior was not cheap, but what I paid was fair.
Even if you do part of the work you should expect to lay down at least $1500 for a professional shop to do a full custom interior - for us in socal.
On the other hand the more simple the work, the less the cost. A few hundred dollars can get you a carpet kit from one of the board vendors and you can put it in over a weekend.