BTC being totally transparent, you can just calculate:
If the miners want to make the same amount they do today, at a price of 500$/BTC it would work out like this:
1 block (25 bitcoins) created every 10 minutes -> for each minute, 2.5 bitcoins are created X $500/bitcoin = $1250/minute.
So every minute $1250 in fees needs to come from transactions (instead of the block rewards today).
Using 5 transactions/sec as maximum speed (with current block size), 300 transactions each minute.
$1250/300 transactions = $4,16 per transaction.
Of course this changes as the metrics change:
2x higher BTC price -> 2x higher fees
2x more transactions per block -> 1/2 fees
Are you sure it's that easy? Clearly the metrics you state do influence transaction fees. But there are also a number of other factors that make calculation a bit more complicated or outright impossible.
For example, one has to consider investment costs in hardware and the efficiency of the hardware. Also cost of electricity is important. I think as we head into the future, efficiency will improve further on all fronts due to increased competition - that might even include use of heat generated by miners for industrial purposes. Therefore I expect that the cost per transaction will increase far less than a simple calculation based on number of transactions, reward, and blocksize might suggest.
(Btw. I think that 2x higher BTC price leads to half the fees in BTC and the same fees in fiat.)
ya.ya.yo!