hi James.
I have just held a large exhibition, and I needed frames as well.
surprisingly, I found very nice - simple frames in..... IKEA(!!).
Real wood - glass - and not too expensive ( ups - I just checked the prices in Australia, and there they are much more expensive that here... (your price for a 50x70 is 79Aud - here it is about 30...)

Anyway - here's a link:

http://www.ikea.com/au/en/catalog/products/40171311