How much is 40 bytes of Bitcoin blockchain space worth? That depends on supply and demand (and the value those bytes can carry). Satoshi never solved this problem and just used subsidies. There ought to be some proper process by which it is determined how much a certain transaction should cost. Bitcoin's design does not allow for this properly and all Altcoins have followed this path. It has to do with nodes being able to leaving the network at will. The nodes should be obligated to stay on the network for longer, so that the overall fault-tolerance can be measured accurately. Interestingly also to observe that one can maximize the total value, by allowing financial derivatives to be processed. Instead of sending and receiving coins, the far more profitable market is to allow arbitrary value to be transferred.

Transaction fees are likely too limited a concept, since they only reward short-term behaviour on a per-block basis. I suggest something like provider fees which can be made dependent on other factors also, in particular the number of other nodes, current demand and transaction type.