It was costed in the 1960s-70s as running into the trillions, but such designs were full vacuum linear motor systems
Hyperloop makes a number of pragmatic/practical changes (such as allowing enough air in to provide air bearings, which in turn reduces engineering costs of suspending the capsules dramatically). Whether it's fully economic to do so is another matter.
My personal belief is that it will prove uneconomic for passengers unless longhaul freight is also carried and that in turn requires large tube diameter to handle intermodal containers.
In all liklihood it would be easier to build a hyperloop in Europe than in the USA, for the same reasons that HSR is all over the place here and non-existant in the USA. Ditto Japan or China - I think the current japanese maglev design is likely to prove uneconomic long-term due to friction issues and the sheer cost of maintaining an uncovered track. (birdstrike?)