The paper introduces a modal logical system for reasoning about knowledge in which information available to agents might be constrained by the available budget. Although the system lacks an equivalent of the standard Negative Introspection axiom from epistemic logic S5, it is proven to be sound and complete with respect to an S5-like Kripke semantics. Categories and Subject Descriptors I.2.11 [Distributed Artificial Intelligence]: Multiagent systems; F.4.1 [Mathematical Logic] Keywords modal logic, epistemology, axiomatization, completeness General Terms Theory, Economics