Just been to the ASDA store in Astley Bridge, Bolton and they have copies of Alien Quadrilogy DVD BoxSet for just £10.
The boxset contains all four Alien films, together with a bonus disc for each film. A final bonus disc contains additional material on all four films.
That's nine discs in total- just over a quid each.
Next cheapest is Play for £24.99 - most other places still charging £50plus (including ASDA Online!)