From support e-mail:
Q: 1. Can DPlot find the area between two curves? Area under a curve?
A:No and yes, mostly. You can embed the area under a curve in any text entry (title line, axis label, legend, etc.) with $AREA(n), where n is the index of the curve.
Since you can embed equations in those same entries and also embed text shortcuts like $AREA in equations, you can indirectly report the area between two curves with something like "$=($AREA(1)-$AREA(2))". This will work fine as long as the curves don't cross. If they DO cross then you can get the area with a bit of manipulation: Use Generate>Y=f(X,Y1,Y2,...) with Y=abs(Y1-Y2) (to find the area between curves 1 and 2), then $AREA() on that new curve to find your answer.
Q: 2. Why is DPlot so much cheaper than other programs such as UniPlot or SigmaPlot?A: Good question :-). Both of those programs will do a few things DPlot will not. Whether that difference is worth 10+ times the price is something users will have to answer for themselves.