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

Yeah well, most of us get paid in fiat and buy things that are priced in fiat...so unless bitcoin has an actual problem that it's a solution to, it's not going to get widely adopted.  It is useful as a store of value, like precious metals, but as a currency it absolutely sucks.  That's my opinion.  I would much rather spend dollars than bitcoin, and it's much, much easier to do so.