I am sorry to say that the code I came up with doesn't use pdegrad at all. But maybe it will satisfy you.
I think the easiest way to estimate the surface area is to add up all the triangle areas that make up the surface. Each triangle is represented in the (p,t) data structure that you can find described here: http://www.mathworks.com/help/pde/ug/mesh-data.html
To find the area of a 3-D triangle whose point coordinates are point = (px, py, u), make vectors v1 = point2 - point 1, v2 = point3 - point1, and use the formula