The spring constant of the Vernier Dynamic Carts with plunger is somewhere between 3.5 and 4.2 N/cm.
The spring constant on the plunger of a Sensor Cart seems to be about 2.6 to 2.8 N/cm. Ironically, everything you need to measure the spring constant is included on the cart, so it is easy to measure. Just turn on the Force and Position channels, put the plunger out and against a solid object, and press on the force sensor to push the cart and compress the plunger.
On either of these, there is a fair amount of friction and sticking (aka stiction) so that any measurement is at best approximate.