Having just checked, MemoryMap charge £69.99 for first issue per chart (including the reader software) and £50 for updates.
To upgrade a complete set of 1:500,000 and 1:250,000 from MemoryMap will therefore cost you £150 (+p&p) and the paper version from Transair will cost you £153.89 (again +p&p).
The problem really is why the 500,000 and 250,000 are packaged together. I think most of us would prefer to only buy the 1:500,000