Technical Debt: Refactoring vis-a-vis Starting Afresh

Before opting for new software (in preference to refactoring the old software), I would recommend you compare the economics of the two option. My rule of a thumb for the calculus of introducing new enterprise software to replace legacy software is straightforward:

For a period of about two years, assume the run rate for dev/test/support will be 150% of your current investment in development, test and support of the old software.

Please note that the 150% figure is just the expected run rate for running the new software alongside the legacy software. You will need, of course, to add the cost of acquiring/producing the new software to the economic calculus.
(Link: Technical Debt: Refactoring vis-a-vis Starting Afresh)