I use a lot of local limestone. It's pretty cheap at $0.10 - $0.20 lb. But before you beat yourself up over using live rock read this report by the UN:
http://www.unep.org/PDF/From_Ocean_To_Aquarium_report.pdf From page 9
"The aquarium industry as a whole is of relatively low volume yet very high value, thus potentially providing an incentive to conserve reef habitats and offering a livelihood to coastal communities often living in low-income areas. In 2000, 1 kg of aquarium fish from the Maldives was valued at almost US$500, whereas 1 kg of reef fish harvested for food was worth only US$6. Similarly, the live coral trade is estimated to be worth about US$7,000 per tonne whereas the use of harvested coral for the production of limestone yields only about US$60 per tonne. In Palau, live rock is exported for the aquarium trade at US$2.2 to US$4.4 per kilo whereas it is sold locally as construction material for less than US$0.02 per kilo. Sri Lanka earns about US$5.6 million a year by exporting reef fish to around 52 countries and estimates indicate that 50,000 people are directly involved in the export of marine ornamentals. In the Philippines, about 7,000 collectors depend on the reefs for their livelihood"
Live rock is also a renewable resource being formed in as little as a few years.