Testing in a Sandbox environment does not make calls to data vendors and the responses we see returned in sandbox evaluations are in fact hardcoded by Alloy.
Therefore this does not incur a cost for the client and you can be certain that the calls will not be billed.
We recommend taking a look at the following article, What are Alloy Tokens, to make sure that you are running a Sandbox evaluation. Looking at the prefix of the evaluation token, we know if it is Live or Sandbox.