It's 20$ on Steam for PC. The MAS is usually overpriced on games. Civ IV can be had for peanuts on Steam, even for Mac (I bought it for like 6$ with all expansions and Colonization included, it's 20$ for just the base game on the MAS).
Anyone who buys game titles on the MAS is getting screwed.
Nah, I disagree. "Anyone" that buys "game titles" on the MAS will not get automatically screwed. I get stuff on iTunes and the Mac App Store cheap all the time. The first most obvious strategy is to simply wait for sales. Just like the iTunes App Store, game developers drop their titles from time to time, and sometimes massively. Add titles you want on your AppShopper wish list and let it notify you. Granted you need a little patience though. But just to name a few examples of some titles that I got for super cheap on the MAS during a sale: I picked up Deathspank for $7.99 (regular $14.99), N.O.V.A. 2 for $.99 (regular $6.99), Modern Combat Domination for $0.99 (regular $12.99), and my favorite sale, Trine for $1.99 (regular $19.99), 90% off, and still on sale today! Surely no one is getting screwed with these prices.
Secondly, the indie market on the MAS is extremely competitively priced versus Steam. In fact, just about all available indie games are exactly priced between MAS and Steam. Titles such as World of Goo, Plants vs. Zombies, Samarost 2, Zen Bound 2, Beat Trip and Machinarium are all exactly the same regular prices between the MAS and Steam.
Ok, so those were just the indie titles. Yes, big titles, such as Batman, Bioshock, and Borderlands are all overpriced than their Windows counterparts and their respective sales just don't match up to Steam's sales. But just like indie titles, they do eventually go on sale, and so you can be sure to expect Batman to at least be a little lower than it is now. You can go even lower, but I had to think a little out of the box on this one.
A combination of iTunes gift cards on sale at Best Buy, my credit card rewards, my bank extra rewards program (for me, Bank of America's "Add it Up"), and the Best Buy rewards zone membership program can net me over 25% off stuff in the MAS or iTunes App store. Put that on top of a sale on a game in the MAS and you can easily get a game over 50% off. I do this all the time and it works great.
Bottom line is though, you are not totally getting screwed for buying from the MAS. It largely depends on what titles you're after (indie vs big budget), and your ability and patience to work the system.
Edit: Case in point: All of the Call of Duty titles just dropped to their lowest prices on the MAS at the time of this edit. Believe it or not, Call of Duty is $9.99, $10 less than Steam, Call of Duty 2 is $14.99, $5 down from the Steam version, and Call of Duty 4 is $19.99, same price as Steam version.