I was checking Rocketmiles for a destination I'm heading to soon, and as far as I can tell some hotels are only marginally pricier through that site while others are considerably more. Probably best to compare the Rocketmiles price against trivago,kayak etc. before booking.

That said, the 3K bonus makes it worth it (to me, at least) to pay a little bit more than the lowest possible price, as a 4 night stay could yield 7K AE points. OK if you're paying maybe an extra $10 a night relative to the other sites, but I wouldn't pay too much more than that.