Looking at the FAQ[1] for it - seems to be 1.70, which is the same as they have on their pricing page[2] for additional IPV4s

[1] https://docs.hetzner.com/robot/dedicated-server/ip/faq-prima...

[2] https://docs.hetzner.com/general/others/ipv4-pricing/

In the server auction the price appears to drop by €2.13 when IPv6 is selected. Which I guess is affected by local VAT.

2€/month/IP address. Ordering without and IPv4 address is 2€/month cheaper