I thought I could get away with the CBT boots for dual-sporting as well, but after busting up my left ankle (hit a rock or stump sticking out of an embankment on a trail) and my right foot (dumped the bike on ice) I've been proven wrong.
I've had good luck with boots off of eBay, advrider.com, or craigslist. Lots of times people will find a good deal online, and the fit doesn't work out right, so they'll drop the price again and resell. For $300 you could easily get both used high-end street and motoX boots in very good to new condition. I currently have some Gaerne road-race boots (bought new in box) and Frey Daytonas (used, but never crashed) for sport riding or the track, the CBTs which I've worn since '94, for street touring, and just bought some O'Neal Clutch MX boots, which I haven't been able to wear, yet.
The Sidi Discovery boots are popular with the V-Strom and GS guys, but the protection probably isn't much better than the CBT boots.
I firmly believe that any one boot that you buy will be severely compromised on either the sporty or dirt bikes, and the worst thing is probably to buy one boot that is compromised in both situations.