Citric acid (synthetic and in fine granular form, but not necessarily food grade) is both, well suited and cheap.
Other less expensive alternatives are acids in high concentrations (but they need to be handled with ABSOLUTE CARE and best knowledge of possible hazards):
Nitric Acid, here the Nitrogen content has to be considered.
Phosphoric acid, here the Phosphor content has even to be more carefully considered, as it may affect calcium uptake.
Potassium phosphate Mono, is less potent, but not hazardous (needs to be handled with appropriate care though),but it adds notable amounts of both, P and K to the
nutrients.