It is not a myth, but it is close to it. United/Madrid sell both 1.4m shirts each year (for the last few years), and are in the first position of 'how many shirts club sell Cup'. If we sign Ronaldo, I doubt that we will sell far more shirts than the other years. Probably 100k more, or at a push 200k more. What wil happen instead is that the likes of Di Maria, De Gea, Rooney, RVP will sell less shirts (yay, all of them were in top 10, together with Falcao, with Di Maria in first and De Gea in fourth spot in EPL). Usually, the same people buy shirts and buy only one. If I want to get a new shirt (right now, it would be of Herrera), then I won't get both it and Ronaldo's. Instead I would choose only Ronaldo's.
Of course, Ronaldo is a big name and will make some people who didn't plan to buy a shirt, to buy one. But it will be a small number. United with all its 300m+ fans still sell only 1.4m shirts (less than 0.5% of fanbase). Ronaldo, or not, it won't suddenly jump to millions.
So, for the sake of argument, lets say that we sell 200k extra shirts (United 1.6m/Ronaldo 1m). For 60 pounds, that will be 12m pounds. Remove the costs of producing and selling them, tax the profit, and then Adidas will get its share. In the end, the profit would be quite insignificant by United's standard (probably a couple of millions or so).
The myth came mostly from Florentino Perez, who is quite good at doing this PR work. Basically for every Madrid's signing, he says that the transfer was payed by the same of shirts. The last one was Rodriguez whose transfer was payed within 48 hours or so. How Perez does calculations, is just the revenue from the selling of Rodriguez shirts, which as I explained above is quite bullshit. Yet some of the ignorant Madrid fans believe it. It was quite funny that apparently both Ronaldo's and Kaka's transfer were payed from the shirts sale, but as
@Rado_N said, the entire commercial income for Madrid that year increased only slightly (IIRC it was only 2-4%) which is expected to happen anyway (and bear in mind, that summer they signed also Benzema and Alonso who were big names too).
While Ronaldo's transfer quite likely will have a goo effect for United, both on the pitch and out of it (making young people all over the world watch United, and those people in a decade will grown up and contribute to Uniteds revenue by buying products/going to OT/buying TV subscriptions etc). But the immediate commercial effect will be almost non-existent.
