struts

Now I think the pasenger side front end is going. Everytime I hit a speed bump or take off from a stop light or sign I hear a knock from that side. Is it like brakes where you should change the pair or in this case all four? In different thread I'm having problems wtih the back struts. Anybody know how much it would cost to fix or should I diy like the last thread. I know somewhat about cars. But this whould be the first time on struts any help whould be great.