A good shock absorber, like the Fox products, will run you close to 300.00. There is, as the guys note, no such thing as a cheap full-suspension bike. The device with the spring around it on the bike you show is not a shock absorber. It is a spring holder. There will be no damping whatever, so every time you hit a bump you will bounce...

Why do you feel you need full suspension? Road bikes traditionally have none; riders simply take a bit of weight off the saddle when hitting bumps.
A much cheaper way to go if you have a bad back or some other problem is the suspension seatpost. Much as with shock absorbers, the cheap ones just have a spring, good ones have a more complex linkage and elastomer elements.
Either will take the edge off a bump.