While joining a state system certainly seems fiscally responsible on paper, one major reason many counties don't is because of insufficient coverage.
The new PA-P25 system has an average of what, 2...maybe 3 VHF sites in most counties so far?
That may work out fine for the PSP and other state agencies, who are primarily concerned with getting 95% mobile coverage or 90% portable-on-the-street coverage, but it's simply not good enough to provide any great degree of in-building coverage for local agencies.
Yes, these trunked systems are astronomically expensive, especially when you start to get into countywide TDMA simulcast systems with data capability. Yes, a lot of money could be saved in theory by partnering up with the state, but only if coverage is sufficient enough. Many local agencies have their own repeaters that are providing good in-building coverage where they need it most, so it makes no sense for them to abandon that setup to migrate to a system that will have inferior coverage because the nearest site is 10 or more miles away.
Now, all of that being said, a good compromise might be for a county to share the state's core. The county would be responsible for building out additional sites to get the coverage to where they need it, while saving a good amount of money by not having to purchase and maintain their own system core. That's a win-win, since the state agencies in that county would also benefit from any sites the county might add.
Oh and one last point...let's not forget that in many states that have statewide systems, they are charging counties and local agencies subscriber fees to use the system, even if the radios are not owned by the state. Those fees can add up to quite a bit of money very fast, and if you do a long term cost benefit analysis, you may realize that within 10 years you've paid the state half of what it may've cost for you to build out your own system with superior coverage! :wink: