One knock against the PC version of Rock Band 4 that's currently approaching the end of its crowdfunding campaign on Fig is that it's not compatible with Rock Band DLC—that is, songs—from other platforms. They're offered on a per-platform basis, so if you buy a track for the Xbox One version of the game, it's only good for that edition. Harmonix has thus added a new reward tier to the campaign that enables backers to purchase the entire Rock Band DLC library for use in the PC release in one fell swoop. It costs $2,500.
The exact number of songs included with the Collector Tier isn't nailed down precisely because Rock Band 4 doesn't have a release date yet, and not all of the existing DLC song licenses can be renewed. (There's a long-running thread on the Harmonix forums that sheds more light on that situation.) By the time it comes out, then, some songs will presumably have been added, and others will be gone. But the tier will include “every single song still available in the Rock Band store when Rock Band 4 for PC launches,” which is expected to add up to more than 2,000 tracks.
In that light the price tag doesn't seem entirely unreasonable, but it's still an awfully heavy wad of cash to fork over up front. It may not matter, though. The Rock Band 4 PC campaign is sputtering: It ends in less than two weeks and has so far raised just $486,000 of its $1.5 million goal—and of that, only $120,000 has come from backers. I'd say it's awfully unlikely that this new reward tier will do much to turn it around.