Good math libraries are already available for free. The GNU'd program Octave---which was designed to resemble Matlab as much as possible---makes use of a set of set of math libraries (LAPACK, LINPACK, ODEPACK, and a few more) which I would like to see as a bare minimum: linear equation solver, numerical differentiation and integration, root finders, and so on. A commercial product containing less than the above is a rip-off, especially if you consider that these libraries are extremely well-tested and thus, while perhaps not the fastest available, exceedingly robust and bug-free. Anyone who has dealt with numerical analysis in more than a passing manner knows how important this issue is.
To answer your question: I would pay for a library which contained more exotic features, such as a good symbolic equation manipulator/solver, a variety of partial differential equation solvers, statistical tools, easy-to-use data format converters, and so on, and so on. It is a very feature-rich field to choose from: just look at the plethora of features available for both Matlab and Mathematica, for example.
