Between Radiant Barrier foil at about $0.13/sq ft
and
Ordinary turkey-wrapping aluminum foil at $0.03/sq ft
To cover 2000 sq ft of a roof's underbelly with commercial radiant barrier would cost, um, about $250.00
To do the same area with heavy-duty aluminum foil would cost about $58.00 for the foil and $1.27 for the extra staples (foil is 18" vs 48" for the barrier stuff).