PHP is free so that's the cheap one. As for the difference, it's pretty much the same concept but written a little different. Both are easy to learn if you know the other.

If you want to take the easy course, learn PHP. First of all, it is open source, and it is much more widely used than ASP, so you will be able to easily find help if needed. There are also a ton of scripts already written in PHP, and PHP is easier to run on more platforms than ASP.

Also, PHP works well with MySQL.