I see people are always using them on cakes. Do they make a big difference with getting sides smooth? Where can I get one... one that won't cost me a bazillion dollars.

It's not that it helps so much with getting the sides smooth... you can do that with your knife/spatula. It helps to ensure that the cake sides are iced more straight up and down (at a 90 degree angle) rather than ending up with a cake that looks slightly pyramidal.

You can get one at Bed, Bath & Beyond:
http://www.bedbathandbeyond.com/product.asp?SKU=10164168

I won't ice a cake without one. As CWR41 says...it helps wiht that 90 degree angle. And also to evenly distribute my icing in case I have it too thick in one area. LOVE my bench scraper!