>>In my son's company he asks the customer if he/she will allow others to have the same new feature. If it's OK, he/she typically pays one third of the estimated cost using standard consultant price (high rate). If he will not allow others to have the new feature, he pays the full price. When given the choice, almost all customers accept to share the feature (and the cost) with others.
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>Thank you for sharing, Tore. This is a very practical and reasonable approach.

You can just call it best practice and then it becomes legally enforceable.