There's at least one designer in the Jewellery Quarter in Birmingham who can design you a silver gemstone ring for about £75, but I've forgotten the name of the place (read about it in the newspaper a few months ago. So, you didn't say silver, but the Jewellery Quarter may be another place you'll want to look.
My fianceé paid a little more than you're looking at, but mine's a diamond, but...still reasonably priced, and we had a good time shopping for it together...