Judging by the negative response when someone tried to promote a retail fusor, or even a kit on Fusor.net, there is little market and definite antipathy towards such an effort. The amateur fusioneers seem to be mostly tinkerers that like to build their own Fusors. Certainly very much more is learned by such an endeavor than simply buying a finished product.
There are also also significant risks that have to be appreciated. These include vacuum chamber implosions, high voltage electrocution dangers, x- ray awareness and interventions if needed, friable electronics, etc. The liability for a commercial product would need to be carefully considered.
As for cost, there are two types of amateur Fusors
The demo fusor and the neutron producing fusor. The latter is a step up as better vacuum control, a deuterium source, and neutron detection equipment are required.
A demo fusor will give almost all of the benefits of the deuterium burning and neutron producing fusors, except of course for the actual denonstration of fusion.
As for cost, it can be as little or as much as you wish. As a cheep example of a series of demo fusors I have built. The equipment list and cost estimates follow.
1) Vacuum chambers- various stainless steel bowels, mayonase jars, plumbing pipes, and pressure cookers. I personally like the pressure cookers as they can occasionally be found in thrift stores used and they are quick to open up for maintainance or modification.
2) Pumps- a used dual vane roughing pump can be bought used or even a new Harbor Freight pump, for less than $300, perhaps much less. I bought mine off E-bay less than $50, the shipping cost was as much.
A roughing pump like this is adequate for most of the demo fusor projects. If you want deeper vacuums you also need a diffusion pump or turbomoleculer pump. The diffusion pumps can be cheap used. I think I paid ~$100 for mine. Some additional assembly, coupling, etc. may be needed. Turbo molecular pumps and their controllers are best but also expensive. And diffusion pump oil can be painfully expensive, but still under several hundred dollars for a good supply.
3)Valves and piping/ tubing- can be very expensive or as cheap as adequate items (some might deny this, at least for neutron producing fusors) can be bought at hardware stores. I think this category may be the greatest cost for me as I have chewed through various combinations as I try to make adequate solutions. Those brass needle valves, etc. add up.
4) High voltage feed through- These can be very expensive or cheap as spark plugs, epoxy glued contraptions, etc. These are adequate for demo fusors if you don't mind some epoxy derived fumes in your demo fusor .
5) View ports- necessary and can be expensive. Can also be dirt cheap as almost any robust glass will do. Sealing them to the chamber can be an adventure, And X ray considerations must be observed, generally not a problem if you stay below 10,000 Volts. This level of operation is fine for a demo fusor where you are only interested in seeing , imaging, possibly other measurements of the plasma. Actual fusion requires additional considerations as higher voltages are required and contamination may be harmful.
6) High Voltage power supply.-These can be professional machines costing many thousands of dollars, or cheap Neon sign transformers, etc, mixed in with diodes, and other parts. An appreciation of the dangers of high voltage and the design limitations is critical. For neutron producing fusors, more expensive transformers, etc are needed. Used dental x-ray machine transformers is one source, but building a system can be challenging. In this and other aspects the forum contribution is extremely helpful.
7) Neutron detection- This can be electronic neutron counters costing several thousand dollars. Some have built their own with the essential component being the neutron detecting tube. Other components can be built from scratch or by modifying a Geiger counter. Bubble detectors are a commercial product that can be bought for a few hundred dollars. The consists of a contained gel that is like a superheated liquid. A neutron will cause a localized vaporization that produces a visable bubble. These are very useful and almost fool proof. The only disadvantages I know of is that they are not as sensitive as the best electronic counters, and they may last only several years.
Deuterium source- This can be Deuterium gas or heavy water with the proper introduction valves, electrolysis , drying agents, etc, etc.
For demo fusors, no special gas is needed. You can use air or any other gas that is not toxic and can be managed safty. You could use helium like the professionals, or hydrogen, etc.
9) Lots and lots of knowledge about vacuums, pashin breakdown, materials, construction, electronics, etc. that is picked up of nessisity when you build your own. A commercial product bypasses this learning process, and much of the fun. You also gain a more extensive vocabulary of choice words.
If you want to grab a fusor off of the shelf, unless you are aiming for a neutron producing machine, a typical florescent tube or better yet a neon tube or plasma globe does the same thing. Playing with the vacuum and voltage and grids though requires a fusor.