Its hard to get simpler or cheaper than a pair of $60 Nanobeams that will give you 400+ mbit of performance and the integrated 16dBi antennas will help reduce interference from other devices. Ubiquiti also has a Litebeam with a 23dBi antenna that's even cheaper - $49... I don't have any experience with those, but I assume they would work just as well (but proper aiming would be a little more critical with the higher gain antenna)
If your existing routers live in the closets, the Nanobeams can too, you just need to aim them at each other.