Charge client for SaaS development?

Question

Situation:

For many years, ACME corp has outsourced Task B to me. Over the years I've learned the ins and outs of Task A (manual labor). Together Task A + Task B = finished ACME corp product.

So now I discovered an opportunity to optimize their Task A process. But to develop a system to help ACME corp to work more efficiently, I need to put in a lot of programming work hours up front. Maybe 1-2 years to prototype, test and finally deploy, then I'll be ready to confront them with my solution. It'll take another 1 year to successfully have them switch from the old way of doing things to my new method.

So in this situation, what is the proper to way to charge? Should I just suck in everything in the beginning? Or charge them for initial development? Ultimately they will be required to pay each time they use my SaaS to complete their Task A.

Note that my SaaS is only useful for ACME corp, I don't see much use for my SaaS product to the mass market.