Why not make it possible to buy professional features for a pretty high fixed price. When paying that fixed price, no limits whatsoever are imposed on the user, not on the amount of users, and not on time.

The only exception from fixed price would be Kaspersky virus blocker and Live Support, since the updates cost money, that Untangle must pay to Kaspersky Corporation, and Live Support cost support workers salaries.

But this could be arranged that if you buy Kaspersky Virus Blocker for a fixed price, you get the possible to use Kaspersky Virus Blocker for a unlimited time, and then you have to subscribe for the updates. So if you stop paying for the updates, you can still run the Kaspersky Virus Blocker Rack, but it will not detect the latest viruses.

Some suggestions for the Fixed price:
Untangle Pro Bundle: $8250
Kaspersky VB: $3300 + ($100/year for updates *optional*)
PC Remote: $6600
RAP: $3300
AD: $2475
Policy Manager: $1650

I calculated the prices by multiplying the monthly price for unlimited users with 33. Thats the price for 4 years - about a 30 % discount.