If Barcelona is building a new subway line snaking around downtown, how would they cut and cover anything other than maybe a few segments? Between the narrow streets, required depths, the Spaniards' talent for boring tunnels, etc., why would they cut and cover? The lessons we can learn are probably less about technological details and more about finding a way to get things done, finding and maximizing synergies, as TrickyRicky says, wherever possible. They made transit a priority and did what it took to get it done, and they seem to get entire projects done in the time it takes Toronto to make a list of reasons why a project is impossible.
As for other cost differences, I would assume that a new Barcelona line running through areas already well-served by subway lines would require next to nothing in terms of bus terminals, parking lots, kiss'n'ride zones, any kind of large public presence, even, other than entrance huts and a few substations and vents and hatches. This might slash the size of concourses and mezzanines, slash property acquisitions, as well as reduce the cost and shorten the timeline of building the actual stations. Alone, this factor could easily save 5%. Just to compare, the massive underground bus terminal at Steeles proposed for the Yonge extension would add more than $150M to what the cost of a station would otherwise be. 5% here and 5% there could add up quickly.
It has been funny, though, to listen to consultants and officials in the past talking about the Yonge subway extension say they'll keep the budget contained (compared to the Spadina extension...yes, the budget already started out more bloated) and the project well managed by trying to get the Spanish to build the extension, Dragados, etc. Ask them if there's any plan to employ on the Yonge extension any of the architects or engineers or construction companies that are building Spadina, and they say "Oh, lord, no. That's a totally different project." I don't know if the idea is still to try to get some Madrid Magic, but what's the point if they're only going to do so on one transit project?