Valuing postal history is an art. The Scott catalog gives values for several representative commercial solo uses. Their starting point (2017 edition) would be in the $50-60 range for solo commercial airmail uses to southeast Asia.
Also as Patg23 notes, The American Airmail Society catalog (6th ed, v3) notes 16,275 pieces flown and a general value of $3.00, which could also be used as a starting point for estimating a retail value. No doubt quite a few of these were franked with solo Tafts.
Using either starting point, one might add or subtract for special circumstances. Unfortunately, being a contrived philatelic first flight cover, with cachet struck over the address, and with significant damage and seam-glue stains would decrease the retail value significantly. It just lacks eye appeal. It would not be in "exhibitable" quality. I would expect a retail value to be much closer to the AAMS catalog than Scott.
As a tangent, consider this domestic solo use dated April 28, 1956:
0.03 first class letter
0.07 return receipt fee
0.40 registry fee for up to and including $5 indemnity.
0.50 total paid with solo use.
Scott does not have a specific domestic solo use listed, other than "on cover" at $10 which would be expected to be with other stamps. Perhaps not as sexy as a foreign destination and on a legal sized envelope, but it is a commercial, non-philatelic use. Definitely an art!