The online travel agency battle royale is on. Just a few days after Travelocity and Expedia eliminated their booking fees for airline tickets, Priceline, the first agency to cut the fee, is fighting back with some copycatting of its own.

The agency is now offering price guarantees that mimic Orbitz’ “Price Assurance” for airfare and Travelocity’s “PriceGuardian” for packages:

That’s why starting today [we're] backing up every Flight AND Vacation Package purchase with Free Pricedrop Protection for orders booked by June 1st. Now you, and your users, are automatically covered if prices drop before the trip!

Up to $300 Cash Back if Flight Prices Drop:
If another priceline customer books the same flight for a lower price, we’ll automatically refund the difference in cash…up to $300.

Up to $600 Cash Back if Vacation Package Prices Drop:
If another priceline customer books the same vacation package for a lower price, we’ll automatically refund the difference in cash…up to $600.

As I’ve expressed before, I don’t think that a price guarantee that relies on another customer booking exactly the same itinerary is worth that much, unless you’re booking a really, really common route (and, in the case of a package, a midrange mainstream hotel). But hey, if it’s not costing you anything and doesn’t take any effort on your part, why the heck not.

Bottom line: The competition for your business is heating up. Who’s next?