If being a developer is around £100 a year and they get free Lion downloads, I'd say it'd have to be less than £100. My guess is around the £80 price range, like Leopard was.
Developers do not receive the commercial release, yet in the past OS X development cost a lot more.
Apple Drops Mac Developer Program Cost to $99
Apple revamped its Mac OS X Developer Program late on Thursday, and the most notable change was a substantial membership price reduction. Instead of paying at least US$499 annually to participate in the Apple Developer Connection Program, memberships now cost only $99 a year.
Since the launch of the OS X App Store, Apple planned on lowering OS X development membership in order to entice more developers, bringing in more $$$ and popularity to its iOS-esque OS X App store.