If you buy a domain somewhere, and it costs 1€/year for the first year and then 70€/year but you transfer the domain to another registrar before the year ends to another domain registrar where it costs 20€/year, how much do you have to pay after the first year?
It depends on the registrar. Typically the price will be similar or a little more than the new registration or transfer rate. Most registrars also provide a pricing table for the 3 common actions - Create/Register, Renew and Transfer
To find the lowest price, you can visit http://domcomp.com. Search for the domain extension/TLD for your domain and check the prices.
how much do you have to pay after the first year?
If the registrar you transfer the domain to charges 20€/year (for renewals) then you will pay 20€ after the first year.
Some TLDs (notably
.com) require you to pay for an additional years registration at the time of transfer, which then extends your registration period.
However, be careful of registrars that offer low initial registrations and then charge you an additional "admin" fee to transfer out. Check the T&Cs.