If a Dapp is to be self-sustainable, one approach would be for everybody using it to include extra Ether in their transaction to pay for mining fees.
Let's say you had a simple Dapp such as one that doled out pocket money to your children. If you put 100 Ether in there, the children could withdraw 5 Ether per week, (depending how they had behaved!) and you'd have to top it up when it was running out. Let's assume that 0.1% of the deposited Ether is retained by the contract to pay for fees, so really you're getting 99.9.
My question is - since the computation cost of the contract in gas is fixed, and the multiplier fluctuates depending on supply and demand, is there any way for the contract to vary that 0.1% depending on current market conditions? Like by reading in a weighted average of all the multipliers accepted by the miners in the previous block.
Space in the blockchain would be a scarce resource - so let's say more important and resource-hungry dapps were gaining in popularity. It seems like the transaction cost in Ether of a fairly trivial app like Pocket Money would change over time.