From previous experience I have to say supermarket version of posh is usually a false economy - it doesn't often wash well. Tried several types and they all went funny shapes.
Have you tried clearance sales for the big department stores? The best stuff we got was from Debenhams and House of Fraser. Both do massive discounts when patterns and colours are being changed. I live in the poverty-stricken North so we do get better sales up here, but my superking sized fitted sheets were £10 each for very good quality.