The Jet City JetStream ISO is presumably only $300 (if you can find it). It's an isolation cabinet.
This video demonstrates about a 15dB SPL difference.
This video claims a 40dB SPL isolation.
I suppose my biggest concern is making a fairly "boxy" sound with it. Also the Eminence speaker is just never going to be quite as cool as my Celestion Alnico Blues.
But the price sure is right, isn't it? And if you need to rock out at home (I do) it could make a big difference. (And again, the second video didn't post the first time. It just got deleted from the html when I pressed "publish". Sigh.)