<p>I'm looking for something containing similar functions to Matlab’s financial and financial derivatives toolbox but don’t have the cash to spend on matlab. I would appreciate any info on free or open source libraries or programs that will let me easily calculate interest rates, risk etc.</p>

How about the
Octave
financial functions?
http://www.gnu.org/software/octave/doc/interpreter/Financial-Functions.html#Financial-Functions
I'm not familiar with the Matlab toolbox, so you'll have to judge for yourself.
GNU Octave is a high-level language,
  primarily intended for numerical
  computations. It provides a convenient
  command line interface for solving
  linear and nonlinear problems
  numerically, and for performing other
  numerical experiments using a language
  that is mostly compatible with Matlab.
  It may also be used as a
  batch-oriented language.