Fully updated HA running on a RPi, the week total cost is higher than the month total cost even though the kWh consumption is way higher, and no there hasn’t been a period of time that the electricity price has been so much in the negative side that it would have paid back that much.
I suspect the energy dashboard is only as accurate as the data you put into it. 
Where are your costs coming from?
The costs are coming from the Nordpool integration, but it still doesn’t make sense how the single week cost can be higher than the month cost, of which, said week is part of. I checked the cost history and it is shown to be on the positive side the whole month with a day or two without data.
If you want to see more about the cost, find the cost stat_id (should end in _cost), and plot in a statistics graph card. That should give more detailed view of what’s going on for cost in the statistics, can narrow down the problem.
The only entity with cost in the name was the one my energy meter is providing, but i don’t use that data, that’s for the meters own app.
Here is the configuration for the Energy page and the data it gets from NordPool
I believe that cost entity is created by the energy integration, not from your nordpool integration.