Just a guess...
I'll take an initial stab at the estimate, but don't be mad because I come upon low performance heads from the early 348s a lot at swap meets. The "791" heads are lower performance horse heads with small valves (as compared to the bigger valved 409 heads). They are specific to the 1958 model year, and as such are of interest to anyone who would like to build or replicate a build from that year. These particular heads have a specific pattern to them that I recall only matches up to a 1958 "W" block, so that makes them a bit less valuable. I would venture to say that to the right person they might be worth $100 (I warned you this might be a harsh reality). That is for a set of unchecked bare heads. Add to that the cost of the machine shop tasks (cleaning, magging, guides, etc.).
I remember what Lamar Walden said they were good for (a "door stop"), but then like a lot of us, he deals in performance-based engine builds. I have several sets of low perf heads, including a set of 1958s, and I'm not letting go of any of 'em. But then I am a collector of these parts...
Some predict that with the Rat Rod guys catching on to the "W look", that these 348 engines may get more attention than in the past.
I'll be interested to see what others think. I do know that the demand for high performance factory iron heads has gone through the roof. That does affect the smaller heads, but I suspect not that much.
Good luck!
TomK
