A recent PAX interview by RockBandAide with Harmonix's John Drake reveals aton of minutiae for hardcore fans, and confirms that Rock Band 3 will allow you to export the RB2 tracks for a fee -- as you could with RB1. Drake states that the the export feature will work for all three consoles, though there are always "a couple songs that get complicated" when it comes to re-licensing. Official pricing and details regarding the export should be available shortly.
Drake also states that while regular DLC will keep the regular price ($2 per song), content made for Rock Band 3's Pro Mode will "probably" be more expensive. Harmonix has "a lot more work to do" on those tracks, and doesn't wish to increase the cost of the regular DLC to subsidize the Pro DLC that's aimed at a smaller group of players.