I'm a noob, but you might want to review/run some of the diagnostics from the tutorial page to make sure there aren't issues with your model and mesh(es).
Looking at the dimensions from the product page, the bounding box for your ruler is 33.7 x 3.8 x 0.3 (cm). That makes the bounding box volume 38.4 cm^3, which would cost about $59 in solid WSF. Your model doesn't fill the box, but that gives you a rough idea of what to expect.
I downloaded your file and opened it with both Rhino 3D and netfabb, and it looks like there may be problems with inwardly pointed surface normals. For example, Rhino computed a negative volume for the basic ruler bar. When I flipped the normals and checked the volume, it came out to be 11.8 cm^3. That part alone would cost $19 in WSF, so $21 may be about right.
There are 729 separate meshes in your file (which seemed like a lot to me): each bit of text has its own mesh. Many of those were also showing inverted normals.
The bottom line is it looks to me like there may be some mesh issues with your file but, if the bounding box dimensions are what you expected, the $21 price is probably right.
Hope that helps,
John
P.S. An idea for cutting down on the price: put "lightening" holes down the middle of the main ruler bar, i.e., between the values of your running feet scale (e.g., 100', 200', ...). Hopefully, you see what I mean...