Magicka and all it's DLC are on sale at gamersgate.com or gamersgate.co.uk. The main game is $3.50/£2.80 and all the DLC is pretty cheap. I bought some of the stuff last night after being unsure of the website, but it turns out that some of the products they sell are just codes that can be redeemed in steam, so I did that. Pretty good experience!
N.B. I don't work for gamersgate, this isn't spam, just think it's a worthy deal that you may not know about!