The ability to measure the satisfaction of (groups of) voters is a crucial
prerequisite for formulating proportionality axioms in approval-based
participatory budgeting elections. Two common - but very different - ways to
measure the satisfaction of a voter consider (i) the number of approved
projects and (ii) the total cost of approved projects, respectively. In
general, it is difficult to decide which measure of satisfaction best reflects
the voters' true utilities. In this paper, we study proportionality axioms with
respect to large classes of approval-based satisfaction functions. We establish
logical implications among our axioms and related notions from the literature,
and we ask whether outcomes can be achieved that are proportional with respect
to more than one satisfaction function. We show that this is impossible for the
two commonly used satisfaction functions when considering proportionality
notions based on extended justified representation, but achievable for a notion
based on proportional justified representation. For the latter result, we
introduce a strengthening of priceability and show that it is satisfied by
several polynomial-time computable rules, including the Method of Equal Shares
and Phragm\`en's sequential rule