So Is This Common To Get A 1915 Expo Cancel On A Common Stamp?

I know this post mark with the panama pacific series stamps is worth a pretty penny. But what about that cancel on this common Washington Perf 12? You would think someone would have wanted at least a nicely centered stamp!

Not a particularly rare expo marking (but much harder to find on cover than on postcard). The Bomar expo catalog didn't actually price it on cover explicitly but you can infer roughly $15-$20 value in the catalog (hard to sell at that price given condition).

For non-experienced folks, the "Model Post Office" is the actually exposition post office station, which is what is referred to in Scott [more generally, do NOT trust Scott values for expo markings]. There are many similar slogan cancels promoting the World's Panama Pacific Exposition and those are (generally) more common, and are not what is referred to in Scott in any case.

I see I was typing as paperhistory was typing!
I don't see a lot of value here. Value is a matter of supply and demand.

In this case, the supply of this particular expo machine cancel is the largest of any station mark according to Bomar's exposition postal markings book, being in use for nearly 10 months. Collectors seeking an example have several thousand or more to choose from and would more likely seek one used with a Panama-Paficic stamp, unless thay are specifically a Washington/Franklin collector. This expo cancel is very common on postcards with the 1 cent Balboa stamp.

For demand, the use of a definitive is generally not "eye candy", regardless of any perceived proportion of surviving mail with definitives vs commemoratives. Beyond that, yes as you noted, most collectors would like a nicely centerd stamp, and on a clean, neatly-opened envelope (or undamaged card).

So while the cancel impression is very nice and fully readable, the stains and rough right end of your envelope detract from its value/marketability. I would see this being a very slow mover in a dealer's $5 box. It has too much nicer-looking competition. But others may have differing viewpoints.

Given its roughly opened condition, and the ordinary stamp used my thought is that the sender may not have had creating a philatelic souvenir in mind as opposed to perhaps just mailing an ordinary letter to someone and happened to do it at the exposition post office? I agree with John that this cover may not have much value because of its poor condition. Collectors normally want covers that display nicely. It is like a stamp with a somewhat high catalogue value but is creased, dirty, discolored and missing half of its perfs. In real estate value comes from "location, location, location" while in philately it is "condition, condition, condition".