The financing effect is the larger part of the value. A lender can credit verified secondary suite income toward a buyer's mortgage qualification only when the suite is permitted, which is why two visually identical basement suites, one with a permit and one without, can support very different offer prices from the same buyer's lender.
Cost and payback vary by scope. A full secondary suite conversion, including a separate entrance, egress windows and a subpanel, runs into real money before it adds a dollar of value, so the decision should be weighed against the rent it can support and against comparable sales of similarly legal suites in the same area, not against a general rule of thumb.
Joel Dyck checks the City permit file on any secondary suite before pricing a listing that includes one. Get your hand-reviewed valuation from Joel Dyck.