How do I fix fret buzz on a budget acoustic guitar in lower tunings?

You have two cheap fixes. First, tighten the truss rod slightly (1/8 turn at a time) if the buzz is in the middle of the neck. Second, if the buzz is near the body, shave a little off the bottom of the saddle or file the nut slots deeper. Alternatively, go up a string gauge—try 12-53 instead of 11-52, or 13s for very low tunings. Some buzz under heavy strumming is acceptable on a $150 guitar; the audience won’t hear it.