Hi,
People purchase 3D models to save time. Consider what you would work for per hour & how many hours it would take to create the model you are pricing. Divide that by 2 so that a purchaser can see that they are saving 50% by buying rather than do the work themselves.
Of course it will vary depending on your skillset the time taken & how much you charge for your services per hour. Hopefully this gives you a starting point - good luck selling!