I doubt that the cheapest source of Live Rock is uncured rock. The cheapest form will be Recycled Live Rock (bought from people in a club forum like this). It usually goes for $3 a pound (average), is already cured, looks good, has MORE life then fresh rock, etc.
Go even cheaper for rock for a tank.... Use 50% recycled live rock and 50% dead rock, base rock, or man-made rock. Travis makes some good stuff and can have it ready in about 4-6 weeks.