Generally the retail cost of any product depends on the cost to make it.
There are three major aspects that affect the quality of motor oil. One is the base stock which can be of varying grades. Off brands sometimes buy their base stock from refineries that didn’t meet spec for name brands. Another factor is the additive package that is mixed into the base stock. A way to reduce cost is to put in “just enough” of these additives to meet API spec and not achieve the protection a name brand might have. The third factor is the viscosity improver. There are varying grades of these also and naturally it would be a place to trade quality for cost. You have heard of oil breakdown, well the viscosity improver also breaks down.
So if you want to take care of your vehicle and quality makes a difference to you, take the guesswork out and choose the name brand.