Mh I get this, yes you pay more on outsourcing but usually you also get a service and an easier time, I would like to understand how much does it actually change..

Surely I'd have to understand how much it's economically worth it. On super basic plans I remember even seeing something like "40€ for 1 year website CMS hosting" (on the cheaper side). Which is the cost of a basic VPS on which you can probably run 100 sites on low traffic... And charge each client 80€ for hosting + maintainance, it's an easy great gain you're getting per site. And maybe add something if they want also email. Great passive income in this case.

But on bigger plans tailored to devs more than to direct clients does it keep being worth it? How much would I spend to host 100 WordPress websites + email on something like Hostinger VS self hosting them all on my VPS?

I guess it also depends on your experience with self hosting, at first I was messing up when doing it, now I understood that most apps really just need mostly the same stuff, and it's mostly all easy set and go. Unless I get hacked or get above some of my VPS limits I don't see big issues coming if I'm hosting 4/5 services for my clients.

I already self host some stuff on my own and rarely have to touch it unless I wanna add features or something like that. I guess with more clients you'll have to factor in scalability and/or managing multiple servers: all great stuff to learn but also yeah def more complex than doing a login and have a nice dashboard with all the services there ready for you..