I have never paid more than $50 for a complete treadle and machine no matter what it is or who makes it. Just because its old does not mean its worth major bucks.
On Singer machines that cabinet style was probably produced in the millions so the rarity is not even a factor. I have 8 or 10 of them here in fair to good condition. The Drawing room cabinets are the ones that bring the money and were not all that popular because of the initial cost. Remember the price of sewing machines have not changed that much they have always been pricey. Back in the 20's the average machine in a nice cabinet was about half of the cost of a Model T Ford!! Parlor cabinets would be next in line for being pricey but I can still find nice ones with machines for around $100.
I do not give anything over $15 to $20 for a used head especially a model 27 (Millions of those floating around also) The only thing that would make me pay more is if the decals and finish were in like new condition. Then I would drop some money on it.
I just bought out an old sewing machine repair shop/dealer a few weeks ago that was closed in the early 1940's. There are lots of NOS (new old stock) parts and machines (still in the original boxes/creates) in the lot along with close to 30 complete treadles and tons of manuals and sales catalogs for several different makers of machines. I am hoping I can fit it all in my 40 foot car hauler and truck with out making two trips.
The cost for everything....$500
Billy