Not true in the UK, McDo really ain't that cheap anymore (or perhaps I'm poorer than I think).

The better burgers made by large commercial chains are more expensive though.

That largely depends on supply and demand in the particular locality. Primary value of McD is that they are global enough that such effects are mostly averaged out and the core products are essentially same globally and the pricing is consistent at least on country level. One thing that recently surprised me is that KFC does not work in the same way and the menu is totally different across not only EU countries, but even franchise holders in same country.