Hi Veilside,
I didn't take the summer courses. I somehow think that they use Hansen & Quinn, but I'm not too sure about it.
About course selection, yes, you can take graduate courses with permission from the teacher of that particular course, and you pay only the LPS tuition for it, which is half the original. Many people take graduate courses, and it's to your great advantage if you do well in them. But I don't think you can cross-register to Princeton and get credit from there. It's two hours or so by NJ transit to Princeton, by the way, so it's not quite practicable unless you drive there.
The cost in tuition and fees etc., which I think you can find on their website, is about $ 2,400 per course-unit. So if you take, say, three courses per semester, it's about $14,500 a year plus health insurance. And living in west philly is not too expensive. You are eligible for loans, but as far as I know, not for scholarships. And if you take three or more course-units (two post-bacs and a graduate), you probably won't have much time to work along the way.