General industry discount is 1 mow with full season. That mow will be the last so it will likely be a joke anyways since the grass will be going dormant. So that puts you at your $3000. On your end it eliminates processing 75 invoices, a bunch of checks, envelopes, stamps, and there is a benefit to getting your money up front. Add in fall cleanup and spring cleanup and go 10% and you'll still be alright. It's a cash infusion, your profit margin will be low, but you'll have a decent part of your seasons fuel bill covered which makes up for the deadbeats that forget to pay in July before they go on their 6 week vacation.
Most of us forget we are working most of the year to cover our expenses. It's only the end of the year that we make our profit.