We've been running SuperHappyDevHouse for over 7 years and we have never charged money for admission. $20k is a lot of money to run an event, ours usually run in the $1-2k range.
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:
--food
----< 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)
--drinks
----< 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.
Here's a breakdown of other costs:
--food ----< 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)
--drinks ----< 50 people >>>> $3 pp = $150 ....bubble tea would be much more pricey, but also much more awesome.
Also note that gummy bears can be purchased in 5 lbs. packs for $16 http://www.amazon.com/Haribo-Gummi-Candy-Gold-Bears-5-Pound/....