In addition to transaction fees, Ripple also has transit fees when exchanging IOUs. Is it supposed to be common to pay transit fees, or is it for rare cases only? How much would the transit fees typically be?
For now, gateways are using transit fees as their primary revenue source. Bitstamp currently charges 0.2% and that's pretty typical.
We hope that those fees will go down over time for two reasons. First, competition and increased volume should bring the fees down. Second, when interest rates start to rise to more historically normal levels, gateways can use the interest on money they hold as a revenue source.
We've been asked to have a "transit fee cap" so a single transaction can't have a transit fee above a limit regardless of transaction size. And we've been asked to have a "fee exempt" flag so gateways can exempt particular accounts from paying transaction fees (perhaps in exchange for a monthly fee). These changes are under active consideration. For technical reasons, they're not as easy to implement as they might seem.