Under-slab insulation vs. slab perimeter insulation
Concrete 4" thick has a r-value of roughly 1 (a lot depends on the weight of the concrete).  2" of XPS foam board has a r-value of 10.  So roughly speaking, using the 2" of foam board under a 4" slab is going to reduce your heat transfer (loss) by ~ 90% (going from a u-value of 1 down to 0.1). 

If you know the area of your floor, you can then estimate the amount of heat flow through the floor into the ground (winter scenario), in BTU's per hour.

Then you can do a similar calculation to estimate the heat loss through the perimeter, compare the two, and see how much the under-slab insulation buys you.