What is the best return on my digital investment?

It depends what do you expect, set your goals, plan how to execute it and when you can expect ROI. The best what you can get are leads I would say.

It really depends on what targets you set for yourself.

Digital marketing is simply a new platform for marketing, hence if your intention was market penetration and you achieved it, that's an excellent ROI. (that can be gauged by pr-defining the number of users you wish to reach etc)