Worthy of further research Curator; I am currently sleeping alternately on ASDA cotton sheets (approx. £8.50) and King of Cotton 800 TC Egyptian Cotton Sateen sheets (approx. £38.00) [admittedly now a few years old]; I'm not convinced that I can tell much difference between the two; I certainly cannot feel the difference when I get into the bed and drift off. You will be able to tell the difference when they are being laundered, but once on the bed and being used, may I suggest that the difference is negligible (even non noticeable)? Admittedly not a true comparison, because ideally all sheets in such an exercise should be new.
Over my life I have always bought quality (buy cheap buy twice) in respect of clothes and shoes etc, but I am not convinced that this policy is worthy for bed linen; again admittedly the ASDA sheets have only been used for a few weeks and the K of C have now been on the bed for around 4½ years (and one of the two K of C sheets originally purchased has now been thrown away / worn out).
Off subject, I know, but similarly, I have now changed my white Christy bath sheet towels for ASDA, they seem okay and wash okay; the Cristy's lasted again for around 4½ years, I am monitoring the ASDA's.