The first thing you need to do is verify that what you have is actually Fostoria glass, in their "American" pattern. Do you have any pictures you could post here on the board?
There are 6 glass makers that I am aware of, who made items in that "tumbling block" pattern, which was originally an old quilting pattern.
The thee pattern names I know are as follows.
Fostoria Glass = "American"
Indiana Glass = "Whitehall"
Lancaster Colony, the owners of Indiana Glass have owned the old Fostoria "American" molds since the 1980s,. They once commissioned Dalzell Viking Glass do a limited reproduction of a few Fostoria "American" items, which are not the highest of quality when compared with the original. Indiana's "Whitehall" pattern was made in two part molds (2 seams), specifically made for that line and no original Fostoria molds were used, which are three part molds (3 seams).
Jeanette Glass = "Cube" (This is a machine made Depression Glass pattern, which is often mistakenly called "Cubist" by some)
I do not know the patterns names used by the next three, but maybe others here do and will be so kind to fill us both in.
Davidson Glass of England
Jobling Glass of England
Crown Crystal of Australia
There may be more glass makers who made a similar pattern, who just aren't as well known as the rest yet. I just recently learned of Crown Crystal, so I wouldn't be too surprised if there were more.
I know for sure that both Fostoria and Indiana did color staining on their glass, so it will be interesting to see which one you have and then fill in the blanks as to whether it is actually rare or just not popular with collectors and sellers, which can sometimes make certain things just appear to be rare, because you don't see them in shops or on ebay for lack of interest in them.