Per the Google's statement: "On April 1, 2017, Google will discontinue sales of the Google Site Search. All new purchases and renewals must take place before this date. The product will be completely shut down by April 1, 2018."

If I were you and you really had already finished the Google Search dev work you referenced, I would pay for service as soon as possible for the duration up until April 1, 2018. Use the service, see how user users are really using it, then use what you've learned to plan for the future.