In case of Iceland, you don’t need to worry. All motorways are free of tolls.
The only payment waiting for you is the tunnel Vaðlaheiðargöng that is 7,4 km long. You can pay for a single trip up to 24 hours before and 24 hours after driving through the tunnel.
Owners of cars up to 3.5 tons need to pay 1500 ISK (approx. 10 EUR) for a single trip.
If you want to save some money, you can pre-pay multiple trips that are not expiring:
10 trips - 12,500 ISK
40 trips - 36,000 ISK
100 trips - 70,000 ISK
You can find the pricelist and purchase the vehicle ticket on the official link, that also provides you with more information about Vaðlaheiðargöng tunnel: