eForce sensor recommendations
10kN is a very large load (a metric tonne). I don't know of any commercial load sensors which can take that load and come in at under $20. (unless you get a second hand one that's rubbish).

Here's my suggestion:



Optical transceiver: These can be used to measure the very small movements as a load causes something to flex. Stick a white piece of paper to the back of a rectangular metal bar. You need to spec this bar so that it moves a little less than 1mm when it's under 10kN load.  Mount the transceiver about 1mm from the paper. Adjust the LED current so that you get a high reading. 



This is a very rough diagram of how it's set up. The scale is all wrong. But you can see that the sensor is set up so that any bend in the metal bar closes the gap between the bar and the sensor.

Other things you'll need are: A way to prevent ambient light reaching the sensor, something to prevent the metal bar crushing the sensor if you place 6 tonnes on the scales.

Pros: easy and cheap to make. Very sensitive. Cons: It's temperature sensitive (like many things), so you'll need to use software to tell the difference between load and temperature. Like kitchen scales, you can simply tare the sensor. Take a reading with no load, then take a reading with the load. Subtract the readings.

Alternatively, use two other IR transceivers as voltage references. Set up one to have maximum light reflected back into it. Set up the other so that no light is reflected back into it. Now use these as the Higher and Lower voltage references in your ADC.