This is the second cheapest I have ever seen for a 4gb MS Pro Duo (£17.99 was the cheapest at Amazon a while back).
Having just bought a PSP, this will be perfect for it.
£17.99 + £1.99 for delivery.
SanDisk Memory Stick PRO Duo cards deliver optimal performance, optimal pricing, advanced security, and guaranteed compatibility with Sony memory Stick-compatible devices.
2% Quidco as well.