Time and materials is pretty common way to bill for services…
But there may be more value created with time and knowledge.
Time and knowledge are infinite, while materials are finite.
Resources that are finite tend to be more valuable due to scarcity.
The value of knowledge ( like information) decreases with time. More people know it and it becomes less specialized.
However, the opportunity to discover new knowledge makes it a renewable resource.
Time, on the other hand, is not renewable and is a precious commodity that is at one time scarce and infinite.
Combining knowledge with time creates the possibility for new value to be created.