There is no additional cost for Overflow, just Wealthsimple's standard fees. We charge a 0.5% management fee on investment accounts below $100,000. If you deposit $100,000 or more, you'll be upgraded to Wealthsimple Black and pay a management fee of 0.4% on investment accounts.
Note that the fee structure for Smart Savings accounts is different. You can read more about this here.