If I purchased a vehicle in sept 2022 (last tax year,) but only started using it this tax year 2023. What opening balance or addition during the year cost do I use?

You use the fair market value of the car when you started using it.

Thank you for choosing TurboTax.

Even if it was COVID and pre-owned prices had skyrocketed? The car was worth more 3 months later than when I purchased it. 

Yes, you use the fair market value when you started using it for business. The rules haven't changed due to COVID or inflation.