Good rocks, like good trees, can cost a fortune, or can be free. It all depends on circumstances.
The rocks you see in bonsai books are almost always Japanese "ibigawa" rock. They are prohibitively expensive here in the states, IF you can find them and IF you can find any worth buying. Most I've seen tend to be on the small side and can run well over $100. The rocks in aquarium stores are usually pretty generic and have been "enhanced" with carving--usually be someone with fish in mind -- wierd looking holes, etc. They are sometimes attractive, but mostly, they're just weird and unnusual looking when placed with bonsai...
If you are patient and know what to look for, you can find good rocks literally in your backyard or wherever you go. You're looking for jagged (not smooth), uneven, but attractive forms, as well as some "age." That means look in areas where rock has been exposed for hundreds or thousands of years (not at all as uncommon as you might think). Here in Va. I have found decent large rocks lining parking lots in the Shenandoah Valley where they had been moved when the parking lot was a pasture, or along stone fencelines in rural areas...