Brake fluid doesn't like water at all, at least on bikes, and causes brake fade and lowers the boiling point, I don't know what difference a couple of spoonfuls would make, but if it was mine I'd have it changed.
I'd imagine water in the brake system would corrode it from the inside out, and cause grief longer term.
As for cost, I guess whatever the price of the brake fluid is, plus an hour of labour for a mechanic. £150?