If someone entered the US legally overstayed for eight years and married a US citizen how much would they have to pay in fines?

Nothing they need to file for another visa in their country to come back.

The person would need to leave the US voluntarily or be deported.

Even though the original entry was legal the person is now out of status and considered unlawfully present.

A person who overstays a visa or is otherwise in the US unlawfully for 12 months or less is subject to deportation and barred from applying for reentry for 3 years.

A person who overstays a visa or is otherwise in the US unlawfully for more than 12 months is subject to deportation and barred from applying for reentry for 10 years.

The exception is of said person qualifies for asylee or refugee status.

Foreign nationals legally married to US citizens are given the opportunity to plead a case for "hardship" if they have children and are gainfully employed.