Any reason as to why 28 Years Later cost so much to make?

I mean from what I've read, apparently most of the film was filmed with a fleet of iphones?? Obviously production alone is a huge chunk, but surely special features can't be that pricey, or can it?? Aside from production, I believe from what I've read and witnessed is that the marketing campaign was pretty aggressive so it must have cost a fortune..plus casting must have been at least a bit pricey considering that Jack O'Connell and Ralph Fiennes make some appearances albeit brief.

What do you guys think?