I'll bite. For your 1950 Singer 66 that needs to be rewired (at least the controller end), I don't care for the 'Godzilla' finish. It also has a ratty base, difficult-to-remove tape residue on bed, and a missing slide plate; all 'minus points.' The biggest negative is that I don't need (want) another 66, so MY 'value' would be $10-20.
For your non-reverse 1910 Singer 66 with good decals but not in a Singer base, the spoked balance wheel would probably say that it was converted after-the-fact to electric. From what I can see, the motor does not appear to be date comparable to the machine; but I believe Singer sold electrified 66's at that date. If I wanted another 66 (to put a hand crank on it, for instance), MY 'value' would be $20-30.
Valuing a machine is like asking someone the value of an old Lincoln penny. If I am not a collector of Lincoln pennies, the value is $0.01. If I am a collector of rare, old Lincoln pennies, then the value to me would be something great than $0.01 if I wanted one with that particular coin's unique characteristics (date, condition, rarity, etc.) For both old pennies and old sewing machines, millions and millions were made, so it really comes down to "beauty (and value) is in the eye of the beholder."
John Thomas in NC