I think there are levels:
"Antique"
"Vintage"
"Rare" :D
"Featherweights"
"Made in 'mass production' quantities" ---- the 66 falls into this category for me.
"Poor machine, nobody loves you"
"High end modern machines"
"Plastic junk"

Obviously not in any value or order.

Singer 66's are very good basic machines. They take class 66 bobbins that are easily available and standard sewing machine needles. Other than really early ones, they also use the very common low shank presser feet. I love mine. I paid $20 for it at a thrift store. I also passed up two treadle 66's in easily restorable condition at $35 each. As others said value is impossible to determine. As far as any collector value goes, Singer 66's were made for over 50 years. There are tons of them out there. Early ones in good original condition will bring a little more money than beaters but they are not rare at all. Instead of looking at the money your friend would probably be happier if he looked at it as time spent with his daughter.
Rodney

Steve,

I hear you. However that Barracuda wasn't made near as long nor in as many numbers as sewing machines. So it's value is also derived from the low numbers surviving.

Not something you can say about a Singer 15, 66, 99 etc etc.

As for not answering peoples questions about value, I usually don't because I don't know the answers. I can relate what I paid for my machines, but that varies from location to location, time of year, peoples attitude and like that.
So usually I don't say much other than to say it's a good price and I'd buy it if I were there, or not.

Joe