TalkinPeace2, I take my hat off to Oasis for building a new school so inexpensively. At full capacity ours will educate 840 children ? 120 in each year group from 11-18.
I'm not sure it is any cheaper to enlarge existing schools. You'd think it would be, but it isn't ? at least, not always. For instance, Ealing Council submitted an application for £300m in BSF funding, £75m of which was earmarked for extending the borough's existing secondary schools by five-and-a-half forms of entry.
Another factor to bear in mind is that enlarging existing schools is extremely unpopular with parents, mainly because it means reducing the amount of play space per child (that's assuming you're adding classrooms, but not enlarging the site's total footprint ? which would, of course, be even more expensive). At my local primary in Acton, for instance, the parents are absolutely up in arms about the fact that it's about to be enlarged.