The standard source for cachets is the Mellone's Planty Photo Encyclopedia of Cacheted First Day Covers. Your covers will be found in Volume III 1930-1932.
The design depicted is from an unnamed cachet maker and has a Planty cachet number of 704/15-22. The value listed for the cachet is $25.00 per cover, so that would give a CATALOGUE value of $300.00 for the twelve cover set.
That being said, as others have pointed out, the FDC market isn't what it used to be, with the exception being some of the rare photographic covers by Beazel or the hand-painted covers by prominent cachet makers like Dorothy Knapp.
I've been working on the National Parks Series, which was issued a couple of years after the Washington Bicentennial series. While the covers CATALOGUE $15-$60 apiece, I've been picking them up on
ebay for $2.99 each - usually. Sometimes, I have to pay more, especially for the more esoteric cachets (not the big name makers of the era like Rice or Ioor).
The big thing, which no one has mentioned, is condition. Most of the envelopes of the period used a glue which, over time, tended to become very dark and would bleed through to the front of the cover. This migration and staining destroys the value of the cover. I will not add any cover to the collection which exhibits that kind of deterioration. One of the ways I keep the covers I have from developing that is to insert a clean glassine inside the cover. This prevents the back of the cover, where the glue was used, from migrating to the front. If your covers are open and clean, it would be a good idea to do this to help preserve them.