Comment consider an open source ERP (Score 3, Interesting) 209
You can throw good money after bad, and you probably will.
If you want to have an alternative, you could do worse than look at Oodo (formerly OpenERP) it is a python based, AGPL licensed ERP package that is modular with a sensible API that is growing an even more sensible API. It is not without it's problems, I wouldn't sugar coat it, but if it is broken, you own all the pieces (http://odoo.com source at https://github.com/odoo/odoo) and that is priceless.
Depending on your specific requirements it might work great, or might be a bigger pain in the ass than your proprietary mess. Like I say, you will almost certainly take the path of throwing good money after bad, but for anyone else at the front end of a decision, the business value of Free Software is huge.