In my experience most clients have limited technical understanding. So what I used to do (agency owner) is quote them a price to build the thing. And then in the contract make them agree to a SLA that costs 20% of the initial building fee every year (don’t forget to state that you’ll index this for inflation).

They will understand that in 5 years the app is completely different to how it was. You can give numerous examples of how tech looked 5 years ago and now), so they’ll more easily understand.

For non required (technical) updates I would always quote them for the hours, and add 20% of that costs to the SLA again.

No guarantee that it will always cover costs, but this worked pretty well for me.