First off, welcome to Audiokarma.
Second, questions of valuation belong in the Dollars & Sense forum. I'm sure this thread will get moved there sooner or later.
Third, RS-4b's are excellent speakers. I bought a pair brand new way back in 1987. They were my first pair of 'real' speakers, and are what started me on my lifelong love affair with Infinity speakers of that era. You can read more on them, and many of the other vintage Infinity speakers, on the
Infinity Classics website.
Value wise, RS-4b's are usually listed anywhere from $150 to as much as $500 for a pair. Realistically, they don't usually sell for much more than $250.