what’s the ongoing costs on the bmw like? i assume mx-5 cup is not hugely different from spec miata, which i race now.
Running costs for me are ~$500 / 2-hour day on 100-TW tires. These cars aren't exactly light, so they eat through consumables. Tires are the largest part, then repairs, then brakes. This doesn't count fuel.
If I had to guess for OP in PWC, 50-100% more, mostly driven by slick costs.
EDIT: added fuel note