Most hackathons, like the Greylock one mentioned, are organized with large budgets in hopes of bringing in talent to find recruits for their companies.
Independently organized hackathons like AngelHack need to charge to ensure long-term sustainability of the event and ensure you're getting certain amenities that you should expect from a quality hack. If you've ever been to a Facebook hackathon you can tell they spend easily 20k between setup, employees, food costs, travel, and prizes.
If PenApps isn't charging yet, then that's because they are being subsidized by UPenn, which is an awesome luxury that most potential organizers will not have.
Well, when you have an awesome event and you're fully transparent about what's going on and where the money goes, things like sponsorship get pretty simple :) Everyone involved gets some good value out of PennApps (which is why you guys have absolutely no problem finding sponsors or funding your event).
As long as you have a venue, Hackathons are not terribly expensive. They typically cost anywhere from $500-$2,000 most of which can be financed by sponsorships. PennApps does have the luxury of being housed at Penn, and the organizers have subsidized use of the rooms there. Given the community that has formed in various tech cities around the world, it shouldn't be hard to find a venue for free or a reasonably if you look around.
Here's a breakdown of other costs:
----< 50 people
>>> (3 Meals at $6 pp) = $300 (sandwiches, pizza, bagel breakfast)
>>> (3 meals at $10 pp) = $500 (with this you can get a well-priced taco buffet, pizza, snacks, bagel breakfast)
----< 50 people
>>>> $3 pp = $150
....bubble tea would be much more pricey, but also much more awesome.
SHDH is great, but I wouldn't compare it in the same light as the hackathons we're discussing. It's not a weekend long event and you cut corners on getting healthy food, because you have no funding. Not to take away from the great fun of a SHDH, but most developers these days expect a certain level of quality from the hackathons they attend.