I am guessing that car registration must work differently where you live to me. Renewing it for the old car would have zero net cost as you just transfer the remaining time to the new car here.
Around here, you just move the license plate to the new car and you pay a small transfer fee to update the registration but your existing expiry date and plate are perfectly valid. You get 2 weeks to go in and update the info on your file and you have to show a bill of sale if pulled over to prove you are within the 2 week period if necessary. You do not need to get a new plate, or temporary registration before picking up the car, it's something you can do later anytime you like within the 2 weeks.
If you want a new plate it's another fee. If you want to cancel the old one they give you a refund for the unused time minus a transaction fee. Government sets the base fee that goes to them and maximum markup the registry can charge you. (of course never seen a registry who doesn't charge the maximum unless you negotiated it down for a bulk fleet deal) They even stopped giving you stickers to show the expiry date, it's just in the computer now. (driving into the USA some of the moron cops you have get confused on this point when you have old stickers from when they still did that)