[wickberg]

Part time PhD's are rare - most professors need their grad students to be working on their grant-related research projects as RAs. That work is then used as the basis for your dissertation. As a part time PhD student you'd need to either have your own research planned out (and separate from the professor's grant-related work) which also implies you're likely to be paying out of pocket for tuition. Or find a professor who was willing to accept a slower pace of work on something that's presumably backed by a grant, which is pretty unlikely as the funding agency is usually holding them to strict deadlines.

Part-time Masters degrees are more common (and what I did myself), but come with similar trade-offs and usually need to be paid for by yourself or by your employer. And as you're not committing to being a part of a professor's research group for the next 5+ years, it's harder to find a good match for an advisor and a good thesis.

Co-terminal degrees (B.S. + M.S. all at once, with an extra year after undergrad to do your thesis) mitigate this, but you're going to be out of pocket for that extra year of tuition on top of the bachelor's.

[hocuspocus]

I live in Switzerland where privately funded PhDs are possible (though also rare) and things work a bit differently since tuition is about $500 a semester. Yet, it's typically not a good deal (for you, that is, for the school it's an incredible deal). You're typically hired at a 50-60% rate by your employer, and while that's comparable to a university salary, overall it's a lot more work, you have to deal with your boss plus your supervisor, the school and the company fight over ownership of your work, and you still have some obligations as a student in terms of credit and TA hours. The worst of both worlds :)