I realize that this is not a direct answer, but have you looked at TurboSquid.com?
There, you will find tens of thousands of high (and some low) quality models that are priced anywhere from $25 to $500, some even higher.
TurboSquid has been in business for a long time. I would take a look at what you see at TurboSquid as sortof the "going rate" for models. (even though they are primarily rendering meshes only - many may not be set for 3d printing)
The thing about intellectual property is that you have to "let your baby go". You can't complain if the person buying it might hits a multi-million dollar salespoint. You got your money up front, and the buyer took a "risk" in that they might not have sold any copies at all. If you were to take on more risk, then you could offer the meshes for a royalty percentage of the final sales price. That system can ensure that you make money in perpetuity, but selling them outright can put coffee in the mug today. You must decide how much risk you're willing to accept versus how much reward you want in the long run.
I my self made a tidy little sum a few months back by taking some items in my shop, upscaling and refining them for a larger size, and then turning full rights to those meshes to the buyer. I don't lament it at all.
I also have no general problem (when asked) with granting someone permission to buy a normal 3d print from my shop and then try to cast it into some other material. If my design is good enough for them to use it as the master for injection molded copies at three cents each, more power to them. I have zero interest in funding the IM mastering process. I just like to design the models... I'll usually tell someone .. I want 2 copies from the first batch run, and then 1% of any revenue after the first million dollars you make.
Still don't have any takers on that yet.
From my own personal perspective, I would suggest that you ask the buyer(s) to allow you to retain "by-line" credit on the models. In other words, that they must say "Designed by Mike" in association with the finished items. That way a third party could seek you out to get you to do even more work.
You might also consider not "selling" the models, but rather "licensing" them.. the buyer can use the model for ## of months, but if they stop using the mesh, then after that time you again have the right to use or re-sell the mesh.