Your second method is not a “ballpark” estimate. It is the way to calculate an actual purchase price if the expenses are known. You would use the first estimate as a ballpark when you don’t know expenses and are trying to get in the right neighborhood.
Although, $22.5k expenses on a $75k gross park is only about 30% ratio–probably on the low end. This is common if you are relying on pro forma numbers provided by brokers or mom and pop sellers.