The price of a child seat is no guide to how safe it is. Currently, there is no official rating for child seats, although the Transport Research Laboratory has initiated a testing programme that is awaiting funding from child seat manufacturers. The programme will employ a five-star rating system based on a series of tests that include front and side impact performance of the child seat, as well as its usability.
In the meantime, all seats should conform to EC standard 44.03 or 44.04, and the best way to ensure your seat is as safe as possible is to make sure its fitted correctly in your car, properly adjusted to fit your child, and that you make frequent checks on its condition and fitment. Always replace a seat after a crash.
Even if there are no visible signs of damage, the internal structure may be compromised. Never buy a second-hand seat you wont know its history, so you could be taking a chance with your childs life.