It can't work as a spam prevention measure unless it has some scarcity. And if it has some scarcity, then it has some value. And if it has some value, then it will find its exchange rate.

I think the exchange rate will be volatile for some time. We don't know how useful ripple will be, and we don't know how quickly XRPs are going to be given away.

Assuming ripple is successful, the value (after all of the XRP giveaways have been done) should increase gradually because XRPs are destroyed when they are used to pay for transactions.

A very optimistic exchange rate could be estimated by assuming parity between the "market capitalization" of BTC and XRP. If 21 million BTC have a market cap of $600 million as at present, then 100 billion XRP might have a similar market cap. This would value each XRP at $0.006.