i think the pitch here could be made more enticing. With SaaS you are not only paying for the current product but also for the implied updates and improvements that will follow. So a comparable model would be how much would a box software cost and what would its upgrades cost (as well as how often are they upgraded).

If the saas cost is being imagined for n years than they should also take into account the inevitable upgrades they would have to pay for with boxed software to keep the software productive.

for example we could take adobe creative suite and its upgrades together for a comparable saas offer.