$75/month savings is outstanding. Is that in Ohio? Was that averaged out over the year or during a specific period? So they're generating around 600 extra kW per month I'd guess, and my ultra-rough mental math guesses your system is a 2 kWh (probably 10 200w panels)? The 2kWh systems are around $10k-$13k+ right now (not including installation) for name-brand panels, which would take a bit over 11 years to pay for themselves (just the cost of materials), assuming no maintenance, insurance, or repairs.
The typical installed cost I was seeing awhile back was closer to $15k (for a 2kWh system, which is popular) so it looks like it has come down a little. You can see how at a $15k installed cost it would take 15+ years to pay themselves off, and given solar panels were estimated for 90% of rated generation for 10 years, 80% for 20 years, and generally a 25-year life cycle it didn't make much sense. Given I'm no expert, these figures are pretty rough.
$7200 is pretty good, I'm assuming they were getting the 30% federal credit, plus some Ohio credit/rebate on top of that unless it's a smaller system than I'm thinking. It's definitely getting better.