When you say "real value of R", do you mean as opposed to the imaginary part? If so, then use real (R) -- but don't expect the output to change much.
If when you say "real value of R", you are talking about the R that would result for particular values of x, y, and z, then you can subs() values in to R:
subs(R, {x, y}, {11.3, pi/42})
But if you are evaluating repeatedly you might want to use matlabFunction:
Rf = matlabFunction(R, x, y, z);
Then Rf would be a function handle that would accept 3 parameters and return the appropriate matrix.
