SFH investors: what is your average monthly maintenance cost? Are you actually cash flowing?

I have a few small, inexpensive, but old homes. Generally under 2,000 square feet and worth between $100k and $150k.

I've been at this for about 1.5 years and budget around $250 per month per house, but lately it seems like I'm under budgeting and it's hurting my cash flow. Had a broken sewer pipe which was very expensive, a gas line problem, and constant leaks.

Because I've been doing this for less than two years, I can't tell if $250 is too little or if it's just a run of bad luck that will balance out to $250ish in the long run.

Is it really impossible to cash flow in SFHs/duplexes?