It's not cheaper at all sync ultra is €3 a month and add free is a €20 one time payment
I mean, you cannot really compare monthly payments with one-time payments. Since ad-free is only available as one-time payment it only makes sense to compare the one-time payment prices for each of them. The one-time payment for ultra is ~180€. The one-time payment for ad-free is ~20€. Is that not cheaper?