When you pay 50% to some third party to support your ERP system, what are you paying for? Not for access to upgrades - you are now out of support and frozen, as far as your vendor is concerned. Not for the vendor's software patches. You are, again, out of support and frozen. Not for enhancements to the version you are already running. Not for security updates. Not for anything that you would normally get from a vendor. When you do this, you are cut off from your vendor. To get back in "good graces" with the vendor, you need to pay back support or re-license the software to the current version. So, going to a third party is a drastic decision.
For the honor of saving 50% and being cut off from your vendor, what do you get? You get locked into another vendor, one who is actively disliked by the vendor of the software you use to help run your business. One that will be (or is being) sued by your software vendor. You get frozen onto the version of software you are running - no way to upgrade now! You get some regulatory and tax updates from the third party vendor. You get some phone support for questions you may have about your software.
In other words, you are getting about 25% or less of the value of software support for 50% of the fee, and guaranteeing headaches down the road when it comes time (as it will) to upgrade or replace your package. This is a good deal for the third party (while it lasts), but not for you.
There are a lot of ways to save money in IT. Vendor software maintenance is not one of them. Just bite the bullet and pay the maintenance, and make sure to use your software to the maximum to get full value out of it.