How do I charge a 5000 sq foot lawn
Do you have any other customers? Do you know their sq ft of lawn? If so you should be able to base your price on that. If not, you need to know how long the job will take you and how much you need to charge per hour to stay profitable.
Try plugging your information into this online lawn care estimation calculator
and see what it gives you.
Keep us posted on how this goes for you.