I do not know of any website to help you determine the value. Much depends on the condition of the treadle cabinet (irons and the wood) and the condition of the head (machine). Does it have nice decals or lots of wear and tear? Are any parts missing? Does it work now or will it need a full restoration? Do you have the manual? Accessories for the machine?
From there it is about where you live and whether there is someone who wishes to buy it.
I recently bought a simple style but 74 drawer cabinet with a 115 in beautiful shape for 120.00. I felt it was a good price. If the seller had wanted to wait for another buy he might have gotton a little more but I see them around here between 75.00 to 200.00 and that is an approximate.