The 1957 Chevrolet master Parts manual shows a price of $135.00 for a 1957 261 short block. At that time, gasoline was about 25 cents per gallon and first class postage was 3 cents. So, if a new 261 short block's cost increased by the same multiplier, a factor of about 13, the NOS short blocks should go for about $1755 apiece. I recall that someone in St. Louis paid $178 for a new short block in 1960.
If I had not just recently acquired a 1954 261 short block in "nearly new" condition, and two late-model (i.e., 1959-1962) complete used 261s that turn-over easily, I would get into a bidding war for one or more of the NOS short blocks.
If James Larrowe gets well over $600 (as he asks) for used blocks, who knows what a NOS short block is worth? Whatever the market will bear. The owner should use ebay. Or, as John M suggests, offer $1000 for both and then start negotiating. As my lawyer friends tell me, "All things are negotiable, even death and taxes."
