How will be pricing computed in my case?

We want to use the Unreal Engine for making VR.
We deploy this VR to our client machines (approx 15 Nos).
This will be only one-time sell for which X amount will be paid to us post-deployment.
We may take 6-8 months for development followed by deployment at the client site.

How will revenue be calculated as it will be an only one-time sale?