Great question, I've dealt with this exact same issue. I've found the best way to look at this problem is by analyzing the problem you are solving for this "lighthouse." Is this something they would build anyway? If so, how much would it cost? Anything less than that is probably a win for them. Is this a nice to have, or a need to have for them? Are they price sensitive? How do they usually pay for this type of product/service?

If you are able to answer these questions it should help you figure out the best pricing model (up front cost + hosting, monthly service, per user/group etc..). It sounds like you have something pretty valuable that they want. I think pricing it based on "modification" is selling yourself short. If you solve a big business pain for them, they should be willing to pay you something non-trivial. My hunch modification pricing would be trivial for you.

Sounds like a great opportunity for you and your company, don't sell yourself short. You'll never get a price that you don't ask for.