Priorities determine how fast your render gets more nodes.
The higher the priority, the faster you get to the top of the queue.
This is just a rough estimate. Use your €10,- of free credits to render a test frame. This gives you the best indication of the render costs.
To make your payment as easy as you want, you can choose out of many payment methods: American Express, Apple Pay, Bancontact(BE), Carte Bleue(FR), Dankort(DK), EPS Überweisung(AT), iDeal(NL), Giropay(DE), Maestro, Mastercard, PayPal, SOFORT(EU), Visa.