How could you possibly know what kinds of things are expensive in a universe that can simulate ours?
What matters is not knowledge but probability. Is it likely that something as complicated as our Universe would be simulated?
Is it likely that they would simulate something with vastly different rules than their own universe with such a high level of complexity?
It is possible that the Universe is a simulation, but it is highly improbable due to the difficulty and complexity inherent to doing so. Creating something of this level of complexity for non-simulation purposes is unlikely.
It is of course impossible to disprove it absolutely, but it doesn't really ...
If it is... I hope they do not crash the system with the test.
http://www.huffingtonpost.co.uk/2012/12/12/physicists-universe-simulation-test-university-of-washington-matrix_n_2282745.html
Be sure to check out the actual reseach papers linked in the article! I would have linked to them directly, but the article is full of follow-on links of considerable interest.