Computation time of a Binom matlab (or C) routine
If you can avoid it, you don't want to compute sums like that at all.


If you compute terms like $p^k$ directly, you'll get big errors due to floating-point precision (because your result will be almost zero). 

If you haven't seen it, one common way to handle small probabilities is to apply the logarithm trick: 

$\qquad \displaystyle p^k(1-p)^{n-k}\ \leadsto\ k \cdot \log p + (n-k) \cdot \log(1-p)$.
If possible, apply well-known closed forms for sums to eliminate them entirely. Computing that is less problematic, both in terms of numerical issues and running time.
Failing that, find an approximation that you can tune to the required precision.


And also:


Computing running times with an algorithm is not computable in general. You will have to apply algorithm analysis yourself. After implementing an actual algorithm, that is. See a good textbook or our reference questions for starters.