Fox are the biggest bunch of greedheads. Almost all of their movies come out at $30 or more. Some companies are more reasonable. Warner releases their older catalog titles at $25 retail, which means you can find it for $20 or less if you shop around, and they always participate in those BOGO sales. And there's always ebay - you can get a lot of HD stuff there for $15 or less.
What I've been doing is selling off or trading in my DVDs - stuff that I either don't want, or know I am going to buy the HD version of. So far I have wiped almost half my collection clean, and have used the trades/sale money towards Blu Rays and HD DVDs. I have over 70 HD movies already, and paid very little actual cash (money outside sales or trade) for any of them. I'm running out of stuff I want to get rid of, though, so I'm definitely going to be conservative with future purchases from then on.