Amano uses Suiseki style rock and each rock usually starts at $100 to $500 but if your patient go hiking and go rock hunting but do your research first and check local laws about taking anything from a natural env. aquaforrest has very nice rock for about $4 a pound. if you have a nano go with aquaforrest. i have a very nice very large rock that i found its about 25lbs but if i dont use it it will go for over $100. yeah i know its just rock, but nice rock is hard to find. I may be selling some extra black lava-like rock but much more dense for 1.50 a pound. see my nano tank thread for pics.
I don't know where in CA you are, but if you have a Sepulveda Building Materials, South Coast Supply, or similar rock / marble / tile building center, they usually sell rock for 10 - 25 cents / lb. out of their 'miscellaneous' bins (stuff too small for sale or that has fallen out of the pallets). If you are only taking a few lbs they usually just let you have them.
also find a local nursery that carry bonsai trees. usually they carry nice rock if your close to san gabriel valley area there are at least 5 nurseries that i have visited. all about fish in Long Beach carries nice rock sometimes.
The Sepulveda in Gardena had a junk bin. Leftovers, larger pieces that fell off of pallets and broke, etc. Tons of stuff. Scaped a 60-gallon for about $6 many years ago... I am sure that South Coast has a similar bin.