It is worth noting that a hosted model may very well be priced identically to a SaaS subscription, with the same services included. This is most likely the case if you purchased your solution from a hosting provider (generally your ERP vendor or one of its partners).
ERP Software and Maintenance
One big difference and advantage of a subscription over a perpetual or term license is that maintenance (and its recurring cost) is included with your subscription. With an SaaS subscription, not only are support and maintenance included, but your SaaS provider takes on the responsibility of maintaining the software. This relieves you of the burden of applying any bug fixes, upgrades, and new releases and also assures you remain current and up to date. With an on-premise or hosted ERP deployment, you will either pay your employees to do it or your hosting partner. And if you neglect to do so, you pay in many other ways, including adding risk.
ERP Hardware and Infrastructure
Then there is the cost of hardware and the infrastructure to maintain it. All software requires hardware, whether you “see” it or not. It needs to run on something. It is either running on your own hardware, whether a device (e.g., your phone or laptop), a server in your own data center, or one you access through some network. If it is running on your own hardware, the price you pay for the software might also include support and maintenance (of the software only), but it doesn’t include any cost of procuring and maintaining the hardware. And it doesn’t pay for the installation, the cost to run it (think electricity, heating, and cooling), or the cost of obsolescence. But rest assured, you are still paying for that, whether you buy or lease the hardware and pay your employees to maintain it or write a check to a trusted third party.