My go-to recommendations for Cabs in the $25 and under category are from Washington -- Columbia Crest, Chateau Ste. Michelle and Milbrandt. As mentioned, you're not going to get Caymus quality with these, but they are usually solid wines. A quick search of WS database shows the 2009 and 2010 Columbia Crest H3 Cabernets were rated 90 pts with a list price of $15, and 3 of their single vineyard reserve Cabernets have a list price of $30 and were rated either 91 or 92 pts. Chateau Ste. Michelle single vineyard Cabernets (as well as their Syrahs and Merlots) are usually solid and in the $15-$30 range. I don't recall if I've had a Milbrandt Cabernet (know I've had the Syrah, which was nice for the price), but their 2009 and 2010 Traditions bottlings both received 90 pts with a list price of $15. Their next step up is the Estates at about $25 I think
For California, I typically recommend Kendall Jackson (Grand Reserve or Grand Estates or something like that) as it is readily available. But, I believe that is Sonoma. For Napa, I've heard some good things about Buehler (though been pretty mixed) and Louis Martini. I've had some good luck with Charles Krug in the past. And recently, I opened up a 2007 Gallo Family Barelli Creek Vineyard (not Napa) that I purchased for $30 and I thought it was a great QPR. Side note on that wine -- WS rated it 83 pts, while I think WA rated it 92.