Build quality's going to be a big factor in this. For example, a £15 case might come with fans and everything and seem like an amazing deal. However, it will not be of equal construction to a £50 case that will, in many situations, last longer. Also, any bundled fans with a cheap case will probably be loud and/or inefficient.
It's worth the seemingly excessive outlay at the start because a more pricey case will last longer and be easier to build in (it will have better cable routing and hard drive installation, for example).