I've found that I don't really look goodin sunglasses, and although i like them, i don't have any. If you say you don't look good in them, i wouldn't get them.
If you have a round face, stick to ones which are more square rather than round. Topshop are good for them and are mostly £15 or below.
New Look had some square blue ones in last time i passed......worth a check. But yes, go for biig ol' square ones. :}
If you go into shops like Topshop, River Island etc they should have a range of cheaper sunglasses of different sizes and shapes, even if the fashion is round.