There is no reason for it to be a taller stadium for having overhanging tiers. Sightline geometry would allow the lower tier to be a shallower rake and consequently a lower construction cost per seat while preserving c-values. Thus affording a proportional larger spend per seat for the upper tier(s).
The corporate areas at the Emirates, Spurs and Millenium stadium are all significantly larger than our proposed ones, with better viewing distances and angles, and roomier tread depths and supporting lounges too. If anything, over lapping tiers allow for larger upper concourses on the same overall footprint.
As far as costs are concerned Feyenoord are currently looking at 63k capacity for a 3 tier bowl, with closing roof for approx £400m.... including reclaimed riverfront land. So I don't think cost should be prohibitive if we are spending £5-600m.