When I add an operation as a Reward from staking, it sums in the dashboard the benefit to the total cost of the token.
I.E: I has 1000 units of a token with a cost basis of 10 units. I stake it and I've got a reward of 300 extra tokens, valued in 3 units.
Now I should have 1300 units with a cost basis of 1000, but what I have is 1300 units with a cost basis of 13.
I didn't pay 13, I paid 10.