New Bushes?

I don't even know if that's what i need, but i'm getting a knocking noise from the nearside rear wheel (i think, difficult as the sound echoes around the car). I have to be honest i have absolutely no experience with this sort of thing, and my knowledge and talent are far from whats required to sort it out. Basically i'm going to have to go a garage to get it looked at, can anyone advise on what the problem might be? just so i've got an idea of what i should be looking to pay for new parts .

Many thanks