"I thought the pistons might be pricy, but gosh! That much!" - Gordon
I did a quick search for "linear displacement transducer" - The application I needed one for was quite special (detecting surface tension of a film of phospholipids on the surface of water as a barrier compressed the surface, and therebye first detecting the peak at which the surface molecules overlapped and the film collapsed but then using this tension reference and controlling the barrier so one could maintain a layer of 1 molecule on the surface while dipping substrates to deposit this layer on them) so I thought perhaps I was way out, and chep ones might be available.
Ones capable of what I did them are far more expensive now!... And in a quick scan I only found (among the £150+ transducers) one cheap one at 89p! .. Just had to look into it ! ;-) .. RS components Data sheet .. Even with the data, I have no real idea about its operation (resistive?) but it looks possibly interesting.
Oh, did see one at £4.84 but it isn't the transducer, its a mechanical adaptor for a transducer.
Anyway, there are hundreds readily available from RS, Mouser,Farnell etc.. Search for "linear transducer" "linear displacement transducer" and / or LVDT - I only looked at the first hit -
Fred.