Is there an equation for the cutoff frequency of a chebyshev filter?
It doesn't matter how the filter was designed. Once you have a specific design, you write down the equation for its frequency response, and solve for the frequency at which that response drops to half power (-3 dB), which is the definition of cutoff frequency.

Matlab is very good at finding the roots of polynomial equations.

If you don't need a lot of precision, just read the value off the graph. Since the peak voltage response is 0.5, look for the point at which the curve crosses the 0.35 level (0.5/&radic;2), which is just shy of 5000 rad/s.