Its pretty much fact in the machine shop world that the 010 theory is all made up, but I keep that to myself. We used all different blocks when we built the engines. Core shift and cylinder wall thickness were more of a factor than casting number or 2 or 4 bolt. The 4 bolts were not necessarily weak for a performance motor, just that we were making more an more power and a splayed 4 bolt was stronger than a factory 4 bolt. So if it was a high hp small block, the 2 bolt core was more desirable. This was in like our circle track motors that ran 100's of laps in and out of the gas. But a stock 4 bolt block could go to 600 hp with no real issue. In low hp engines, we even searched for the hecho en mexico blocks because they were lighter. They could handle 4-450 hp and save a few pounds on the car weight. In today's world, the aftermarket blocks have taken over for anything other than a decent street performance motor just because it costs so much in machine work to prep a factory block.
This ^^^^^ is what I've heard and my machine shop claims.
I take this to mean, for a hot street driver, 400hp or there abouts, any old 350 block will work and be more than reliable. For a strip car, they will will work fine also as long as you aren't taking a 250hp motor and putting a 300+ nitrous shot into it.
A stock block LS will definitely hold another 200-300 reliably, but for any thing that is serious, 4digit power, an aftermarket block is the solution. And that doesn't necessarily imply an LS aftermarket block. There are still alot of guys running aftermarket Gen1 blocks to make 1500+. Depending on who you listen to, the SBC head bolt pattern is much, much better for holding a head gasket than any LS due to bolt count and location.
Original question - what is a SBC core worth? Whatever you're willing to pay when you need one.