under $20
under $20, if you don't buy an used book, you can only have a pdf
If you want something balanced with D&D, and probably d20 modern (you'll get a free updates when it comes out), i.e. if you have a hero with a character level 15, you know that level 16 warrior or CR 15 monster are a good match for him you have Four Color to Fantasy:
$6.95
http://www.rpgnow.com/product_info.php?products_id=235&PHPSESSID=8809f845103e6c00f444dd000c343a0a
It isn't exhaustive on powers, but a second book should come sometimes around october that will expand those already included in the book, and you have guidelines in it to creates new one (a system that give the cost of a power based on the level of a spell that would have a similar effect).
So if you are interested in a small book (under 100 pages) that you can easily handle this one is good.
With a similar size, you have another aproach, Vigilance's classes are balanced against each other, so you'll probably use NPC as adversary with this one.
$5
http://www.rpgnow.com/product_info.php?products_id=146&PHPSESSID=8809f845103e6c00f444dd000c343a0a
Now if you want an exhaustive book, you have Deeds not Words at 318 pages, like Vigilance the classes are balanced against each other, so if you want to use other source, you'll have to eyeball challenge, and your character won't probably use material from other sources. (it doesn't prevent the NPC from using them, but I think you'll have some problem if you include a sorcerer in the party).
$10
http://www.allforart.com/cryptosnarkgames/d20/deeds.html