I try to charge by the hour - 2 guys, 75 per hour to prep the property, 100 once the aeration starts. You get an idea after awhile of doing what to tell the customer as an estimate, but I don't like giving estimates if it brings down my bottom line. If your estimate is higher than your hourly wage, the customer has already agreed so it is OK. We just did one, estimate was 300, we were done about 45 min early, I dropped the price $25. We made great HOURLY rate, customer paid LESS than estimate, everyone HAPPY.
Set price by SF is OK, but every property is different and some 5k's will take almost as long as a 10k.
Only seed and fert matter to me when pricing by the SF. Labor is always going to vary.
I like knowing what I'm going to get PER HOUR because it is easy for me to know that I will get my price regardless how long it takes.