The stamp picture above is what some of us would call an "SSA", a shortened name for Small Stamp A. There is a very similar stamp known as SSB. The difference between SSA and SSB may be seen at the top of the Ottoman script of a stamp. An SSB shows a full solid stamp. On an SSA, the very top of the Ottoman script is missing. Timeline falls into the order of SSA (mid to late 50's), SSB (late 50's), 60's 3 Dots.
As ThomFloor recommended, there is more information on Zenstat's website.
Weight plays an important role in the value. But typical 14's from the late 50's (800-900 gm / 800-1000 gm) are usually around $125-$150. You can easily pick them off for less since they often get confused for 60's or 70's. Lighter pairs like 550-725 / 575-750 gm can bring more. The lower, the better as far as value. A friend of mine recently found a paper thin pair at 510/600 - got well over $200 for them. The lowest weighted pair I've been able to buy right are 633/755. I'd value those at $150-$175. The closer to orig factory brilliance - and sounding great....adds value as well.