Well, part time job would imply selling
My guess would be 5% the part of the bill that involves the making the specific interior or architect stuff, but it is an awful exercise to keep two databases for finances (one towards your customers,one to split out the costs of the engine use for the specific customer demonstration vs other income//expenses)
Kinda depends on how you use it too I guess, do you give the customer a ‘game’ where they can walk around to see the interior or do you only show it on your pc//laptop when you are with the customer.