The problem with having a silver or diamond base is not only just cost, but manufacturability. For my main computers, I always lap my heat sinks, even the ones with direct pipes. That's because I tend to buy cheaper ones that are not milled to as high of surface qualities and tolerances.
Sure, you could diamond dust the contact surface but I don't think it would make much of a difference as you're going to spread a THIN layer of thermal paste anyway, and lets face it, even the best thermal pastes marketed to consumer electronics are several orders of magnitude worse than the heat sink material at thermal conductivity, but several orders of magnitude better than air.