First off under the £25-30 mark the steel tends to get really thin, the overall quality takes a plunge too so you'll end up with sharp edges, low tolerances which means when you come to make something it may not fit correctly.
Generally if it's under £20 it's not really worth it, remember ontop of that you've got p&p, the free scan shipping isn't going to apply for something that cheap
Not sure when the last time you bought a case was but even the cheapest come with audio sockets, granted they can often be really low quality but they'll be there none the less.
As to shiny fronts, that's just the finish on the plastic bezels, normally it doesn't really cost any more to make them shiny as it's part of the moulding process and plastic used, probably a little but it'll be pence per case.
You'll find the main cost in cases is in setting up production runs, shipping and the materials, it's one of the reasons cases often share the exact same internals, because the frame is the same they just has different bezel designs and side panels.
ATX or mATX?
A good cheap mATX case the fractal design core 1000 is a great choice for £30
http://www.scan.co.uk/products/fract...e-case-w-o-psu
for ATX I highly rate the casecom K5-9388 at £28
http://www.ebuyer.com/239238-casecom...an-k5-9388blue granted ebuyer only do free p&p on orders over £50, yes it's got an led fan and shiny bezel but they are great quality for the price and is comparable to some £40 cases for features and layout. there is also the K5-7688 for £21 which is basically the same internal frame, just lacks the 2nd included fan and has a different bezel
http://www.ebuyer.com/251582-casecom...nt-fan-k5-7688