A modeller might create a dress in a few hours, at rates from $10 to a few hundred dollars an hour.  To split materials, body parts and make it a conforming figure with the most common morphs add another few hours.

To sell a complete, working conforming clothing item would add a lot of money assuming transfer of copyright.  The person making it would have to consider all the sales he could make by selling it himself.

If you just want an obj file , to rig yourself it would be cheaper.

I believe a vendor sold off some of his product line at about 1200 - 1500 per figure but I could be wrong.

I think you need to clarify what you put into the deal, if anything.  If you had for example some design sketches for products then you could possibly go as partners with an existing vendor.