If you do not plan on sustained high rates of fire, the Harvester is the way to go. They currently run in the price range of $600. (I think.)
That being said, if you are thinking down the road of a semi-auto platform, the budget 30-cal can I would suggest is the AAC SDN-6 in 7.62. SilencerHQ carries them at about the $650-ish range. One caveat is that AAC doesn't give you a mount with the can, so you are looking at an additional $85-$175 depending on the mount of your choice. (Of course, you could always save up up while you wait for the NFA to approve your stamp.)