Are you suggesting the "free" download is just a marketing campaign for the $50 box set? Never.
It's $81 (£40), not $50, and no... Considering that each one is individually made-to-order, I doubt they plan to sell many of them. I think they intend to see how much money they earn off of the "free" download.
Keep in mind that your average "well compensated, established artist" typically receives approximately $2 per "album unit" sold ($1 royalty to the composer and $1 royalty to the performer). And, remember, the vast majority of record contracts require that the artist, not the label, is responsible for promotional and production costs for the album out of "their share" which can significantly reduct the actual royalty payments.
Of course, Radiohead could probably command a large premium (especially considering that their record contract had expired so they are "free agents"). But, even assuming that they pull in double that (so they would get $4 per unit), and that they actually keep all of that money, Radiohead doesn't need many people like me willing to pay $6 (not to mention all the die-hards that'll actually pay the $81) in order to compensate for any "lost" revenue from the people who would have bought the CD but will now pay $0 for the download. Their costs are extremely low. Basically just bandwidth and web design costs, and they haven't yet announced how the download itself will actually happen -- perhaps Bittorrent? And, they aren't losing anything from those who would have just downloaded the music off of Pirate Bay (while getting some good will in exchange by making them not feel like criminals), because they would have paid $0 anyway. Perhaps some of those people will actually pay to go see them live (which is where these artists make the lion's share of their profits anyhow), and perhaps some (like me) will actually pay to support the new distribution model.
Besides, they've already announced that they're in negotiations to release the album via traditional means (CD in stores) with a number of different labels.... The CD should come out in early 2008. And that is the real point of this experiment. They're saying "We don't need you" to the record companies. The current model is that an artist signs a contract with a record label and essentially becomes "an employee" of the label for a set period of time, in exchange for that label's production, distribution, and promotional capabilities.
However, the world we live in is quite different. Radiohead is saying: (1) we have our own studios (which you made us pay for anyway), (2) we have our own promotional apparatus (which you made us pay for anyway), and (3) now we can even distribute our work ourselves. This is about changing that relationship between the label and the artist to one where the artist hires a label for a specific album or product (rather than the other way around). In other words... Why isn't the model: we make the album, we shop around for the label with the best terms (while distributing our work online to those who will pay), then we sell the right to make our album into a traditional CD on a one-off basis to the highest bidder. Next time we make an album, we do the same thing, and maybe there is a different highest bidder.
In other words... Capitalism and competition. Something that has been missing in the music industry for a long, long time.