Regarding cost - The solo album could be made pretty cheaply if JP wanted to do that. A lot of professional drummers have recording setups in their house. Recording guitar and bass at your house is pretty easy. But JP's mentioned in a couple interviews that he finds making music over the internet to be creatively unsatisfying. He likes getting in a studio and recording because it makes for a production that, to paraphrase, is more than the sum of its parts.
I disagree, but it explains why JP isn't doing something like what KM, Jim Matheos, and Gavin Harrison do for OSI.
I agree with your post completely, but honestly it still wouldn't be that costly to JP to put down a solo record in a proper studio. It would just have to be different than how DT does it, I'm sure he couldn't afford to set up in the studio, write the songs there, then start recording. But he certainly could write the songs, send them to the drummer and bassist (whoever they end up being) and then exchange part ideas back and forth remotely. Once they have a clear picture of what they want to do, they could get together for a few rehearsals and then load into the studio. If he uses MM on drums, MM could probably set up and lay down his parts in three or four days, bass would probably take the same amount of time, maybe even less. Then its just up to JP to finish everything else.
Thats a very doable scenario, in fact thats how my band is currently preparing songs that we want to eventually record because its just not feasible for us to rent a practice facility every week, we live an hour away from each other, etc...
All in all, I don't think financial cost is what has been keeping JP from releasing a new solo album. I would suspect that he has been more focused on continuing DT's momentum in the last 4 years (since MP left) than he has ever been before, so it just comes down to time and priorities.