I have dealt with this before. I recommend finding out how much the acquirer pays to acquire customers, users or accounts today. CPCA- cost per customer acquired

Also ascertain how much a client, user or account is worth to them. LTV - long term value

From these metrics you can put some value to what you have. Then trend that out over 3-5 yr period.

The key is to make the buyer part of the valuation. Don't go off and do this and come back with a value. They need to be a part of it for your valuation to be accepted. There are no absolutes in this.

From my back of a napkin calculations that is what Facebook did to buy Instagram.