If you're costing out 6 users,
Perceiva seems to charge $770 for the first year and $720 each subsequent year.
mindcentric seems to be charging about $700 for the first year and $600 each subsequent year. Some of the other HSPs may be less and some may be more -- these are just the first two I checked.
We don't tell HSPs how to price the product. If you can't find an HSP that fits your customer's price range, we regret that they've lost your business.
Incidentally, if you'd like to contribute to the OSS version, we welcome your submissions. I'm not sure that rewriting the over-the-air sync or the MAPI implementation is the best bang-for-your-buck use of your time, but one of the sometimes-great sometimes-frustrating things about open source is that people can work on whatever makes them happy.