The thing about creating your own setting is that it requires a little imagination but usually a lot of time to invest, whereas with a default one, you can take it as given and/or adjust it as you go along to suit current and/or expected circumstances, on a ad hoc or premeditated basis.
The Elestrial Concordat setting took a full session to create as part of play...
Character gen was another session.
Session 1 - we discussed what kind of setting, and the basic needs to modify the generation protocols. We opted for none.
We then generated the systems's mainworlds, and cultures as "no more than one index card" each.
We then fleshed out the rest of the systems using T20 expansion tweaked slightly.
The following session, we generated characters, and the mission. (Which, in that case, was "develop the J2 drive"...
So, really, the setting took 5 people 9 hours each, plus an additional roughly 10 for me to collate. Then 30 hours of assorted art. So, about 85 person•hours. (Slap the average wage of any of the group on, at the time, $12/hr, and you get about $1020.)
Canned settings, even at $50 to $100, are comparatively good value for money, even when you factor in the need to spend time learning it.
My current D&D world has 30 years of sporadic use. And about 30 hours of development - mostly between sessions. The map was randomly generated. The cultures were by taking suggestions. It's grown from play, even tho no two campaigns were set on the same continents. It has a certain depth to it that a canned setting seldom does, and yet, lacks the breadth that canned ones usually do.