How do you know you don't exist in the matrix? And that the true universe above ours doesn't have infinite computing power (or huge but bounded, if you don't believe in infinity.) How do you know the true laws of physics in our own universe don't allow such possibilities?
You can say these things are unlikely. That's literally specified in the problem. That doesn't resolve the paradox at all though.
If you assume that the probability of somebody creating X lives decreases asymptotically as exp(-X) then you will not accept the deal. In fact, the larger the number they say, the less the expected utility you'll estimate (assuming that your utility is linear in the number of lives).
It seems to me that such epistemic models are natural. Pascal's Mugging arises as a thought experiment only if you consider arbitrary probability distributions and arbitrary utility functions, which in fact may even cause the expectations to become undefined in the general case.
Summary: the problem with Pascal's Mugging arguments is that, intuitively, some probabilities are just too small to care about. There might be a principled reason for ignoring some probabilities, namely that they violate an implicit assumption behind expected utility theory. This suggests a possible approach for formally defining a "probability small enough to ignore", though there's still a bit of arbitrariness in it.