A lot of good advice here.
The three rules I follow:
what do I want to make net?
what's the cost to me plus reasonable profit?
what is the competition charging?
To the first, set yourself a reward that you can aspire to, like a new alternator for the Camaro. $ 600. That way you'll be inspired to finish.
To the second, what does the place where you work charge for you? $ 30/hr.? $100/hr.? Takes me twenty hours to build the thing. That's $ 600 to $ 2,000 plus parts (minimal).
To the third, elusive and unreliable.
Set yourself a schedule, draft a short form agreement letter, ask for 50% up front, ask a lot.
"Mr. Douglas, I will charge you $ 900.00 to make this model. I will finish it in 30 days. I will ask for $ 450.00 up front. I will not charge you for the cost of the model plus supplies. I will allow up to two hours to correct things you don't like. I will charge you shipping or find a reasonable way to give you the model. Balance due on delivery."
That sounds like an entirely reasonable set of conditions, doesn't it?
The usual failing in this enterprise is a lack of will to charge what you are worth. Who is more capable of buliding a well researched, well built and dependable model of this aircraft than you.
NO ONE! Worth an extra $ 1,000 right there.