I've bought Singer cams from between $2 and $10 each - there are a few that are rather hard to find. However, if what you have is the basic set that came with the machine (and that set is NOT sequentially numbered) then the whole set goes for about $10 - $20 as they are very, very common.
Though I'm pretty sure the basic set comes with only 12 cams, and there are a total of 21-22 cams - 22 because there is a special one but it only works with the newer group of machines.
I know that 8 and 17 are two of the harder to find cams.
They fit the 400 series on up - for a lot of machines. I have two full sets and several extras on hand.