Pft deterring fees will occur automatically as the block reward disappears. 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 10 transactions/sec as maximum speed (with current block size), 600 transactions each minute.
$1250/600 transactions = $2,08 per transaction.

Even higher BTC price -> even higher fees.

Edit: and if you use 5 tx/s instead of the theoretical 10 tx/s the amount doubles.