When i was a kid i thought my £300 sony Hi-Fi sounded good, now i'm older i know better.
I suppose expectations come into it, but i'm affraid a £50 HU will never sound 'so so good', you should listen to something that sounds good then make judgement.
See, I think it would sound good, based on the following :
I have a Sony BT2600, cost £100, but is really a bluetooth version of a £60 head unit.
It sounds FANTASTIC in my Passat because the stock one was pants, on that same note, for stock speakers VW ones are pretty damn good!
I think what you're referring to as good sound may involve replacing the speakers, which is an extra cost. I know this because a £300 Pioneer doesnt sound any better, BECAUSE of the speakers. So I know the sound which comes out is great, and I'm happy with it, its good. But what you're saying makes it sound like at this budget it shouldnt sound good and isnt worth it.
To the OP, get a cheapish head unit with an Aux in, what car do you have? chances are it will sound miles better than what you have already.
EDIT - Yes, expectations do come into it, but for £50 odd quid you'll get bang for your buck