Absolutely depends on transaction volume.  If the price of 1 BTC is $2.50, and there are no block rewards, and there are 25 transactions per block on average, then I figure a transaction fee of $0.50/transaction will still provide around 1 TH/s of hashing power, which I believe would be sufficient to secure the network if the entire currency is worth $50M.  I calculated this all out in another thread, but that's the result.

I don't think $0.50/transaction is an unreasonable cost to pay - it's still a heck of a lot cheaper than a wire transfer.  And that's basing it off of today's transaction numbers.  5-10 years down the road, when the block reward really starts getting small, we may have a lot more transaction volume.  If we had 2500 transactions per block, we could still maintain around 4 TH/s per dollar price of BTC, and drop transaction costs to around $0.05/transaction.

The yet-to-be-determined factor is how much hashing power we actually need to secure the network.  In my opinion, 1 TH/s is plenty enough to secure the network with each BTC worth $2.50, but the goal should be linear growth in hashing power relative to price from there.  So 10 TH/s if each BTC is worth $25, etc.  Once we figure out that factor, then some hard numbers can be calculated for transaction fees based on current transaction volume to reach a target hashing goal.