I'm in the process of replacing shocks on my wife's truck. Local parts house starts at $50 each up to $75 each. Got a pair on line for $30. $30 for 2 shocks. Gas charged heavy duty shocks.
I'm going to turn into my salesman mode for a bit, so bear with me.
Granted, I've been out of the automotive Buisness for 15 years but the knowledge is still basically the same.
Most shock absorbers, from the factory, have a 1 inch piston assembly and have a general life span of 20k miles. Aftermarket shocks generally are listed as "heavy duty" , and have a 1 3/16 inch piston. Some pick up truck shocks can go up to 1 3/8 pistons. The pistons have a series of valves built into them. This allows the fluid to move from one side of the piston to the other. The more valving, the smoother the ride and the faster response time.
Gas charged shocks are filled with pressurised nitrogen. Generally from 100 to 200 psi. The reason for pressurised nitrogen is this. Old time shocks had air in them. Air mixes with the oil and foams making the shocks less effective. Nitrogen won't mix and cause foam. Thus doing a better job of controlling.