XRPs should not become much more expensive than they currently are. They're basically a fee to create an account, and that fee shouldn't be all that high.
Then why do Bitcoins have any value? XRP will have as much value as the market will assign to them. XRPs have all the properties of a currency. Just like Bitcoin. You are wrong. Here are some thoughts from others:
A few comments...
XRP as a currency.
XRP are not "just" what you use to send Ripple transactions. They are also the only thing you can hold in Ripple which is not an IOU. I think XRP will most definitely be a currency, with a non-negligible total market cap which floats against other currencies. And to understand why you need to look no further than why Bitcoin is/will be a valuable currency.
Bitcoins are valuable because they are in use, and they are in use because they are scarce, durable, portable, fungible and divisible; that is, a good currency. So are Ripple credits:
Scarce - There's an upper bound of 100B XRP.
Durable - You can store credits as private keys, back them up and encrypt them.
Portable - You can send them easily anywhere using the Ripple system.
Fungible - I don't know enough about Ripple but I'll guess all credits are the same.
Divisible - There are a total of 100 quadrillion drops in the system.
XRPs have all it takes to become a good currency, and I believe it will be used as such. A currency also needs to be current, and here psychological factors play a large role - the fact that many of the early adopters don't perceive it as a currency might slow down the process, but it won't stop it.
The fact that, in addition, XRP are what you use to send transactions in Ripple, will help them gain value as Ripple becomes in greater demand.