Hi Denis,
Kevin and Jim have given you some pretty good (if opposing advice). An additional thought would be to look at what you *want* to be doing. If working for them is a goal, then approaching them becomes more attractive. It's also attractive if you want to move on to a different project. You can sell this one and possibly get enough to tide you over while some other venture can be started.
OTH, if you want to stay with this type of project and be on your own approaching them makes little sense. If you sell, they will want a Non-Compete that will keep you out of that market for an extended period of time. Unlike employment non-competes, this will be more enforcable.
If you do sell, look at getting something upfront and also a cut each year. Also, you might want to structure the deal so that if don't actively work on the project (earning you money), everything reverts to you.
Like Kevin said - see your attorney first!! You'll get a much better idea of what you can and can't expect.
Regards