While Apple's latest OS 10.6 -- aka Snow Leopard -- costs a reasonable £25, those Mac users upgrading from the earlier Tiger OS need to invest in the Mac Box Set.

The new Mac Box Set -- featuring Mac OS X 10.6 Snow Leopard, iLife ’09, featuring iPhoto ’09, iMovie ’09, GarageBand ’09, iWeb ’09, and iDVD and iWork ’09, Apple’s productivity suite for home and office including Pages ’09, Numbers ’09, and Keynote ’09 costs £129 and £179 for the Family Pack.

The Snow Leopard Family Pack costs £39.

However, as many users and Mac specific sites have discovered, a £25 copy of Snow Leopard will upgrade just fine on anyone running Tiger on an Intel based Mac.

The snag, doing so breaks Apple's end user licence agreement (EULA), as Alan Eyzaguire, director of software product marketing at Apple Europe -- and Middle East and Africa -- told The Guardian.

"Technically, yes, it would upgrade a Tiger install," Eyzaguire told The Guardian's Charles Arthur, "but in the licensing, no," finally answering a question about the need for Tiger users to invest in the Mac Box Set rather than a copy of Snow Leopard.

Eyzaguire insists that the Mac Box Set has been "incredibly popular", but when asked about how many copies have been sold added, "We don't give numbers."

An Apple spokesman helpfully dug out the precise bit of the agreement for The Guardian:

If you have purchased an Upgrade for Mac OS X Leopard license [which is what comes in the Snow Leopard box], then subject to the terms and conditions of this License, you are granted a limited non-exclusive license to install, use and run one (1) copy of the Apple Software on a single Apple-branded computer as long as that computer has a properly licensed copy of Mac OS X Leopard already installed on it. If you have purchased a Family Pack Upgrade for Mac OS X Leopard license, then subject to the terms and conditions of this License, you are granted a limited non-exclusive license to install, use and run one (1) copy of the Apple Software on up to a maximum of five (5) Apple-branded computers at a time as long as those computers are located in the same household (as defined above), are used by persons who occupy that same household, and each such computer has a properly licensed copy of Mac OS X Leopard already installed on it. The Family Pack Upgrade for Mac OS X Leopard License does not extend to business or commercial users.

You can read it yourself at Apple legal (PDF).