We get diluted. Every investor gets diluted. They may have the right to buy more stock in later rounds to maintain their percentage, but they do have to buy it, at the price of that round.
Nope, YC's equity gets diluted like everyone else's. In fact, while many investors will buy more shares (at the new price) to keep their % ownership, YC does not, so their ownership % when the company exits is probably 3% or less, down from the original 6%.
Usually $11,000 + $3000 per founder. So $17,000 for two founders, $20,000 for three or more. We've also arranged for each startup to get $80k in convertible notes automatically. The goal is usually to give you enough money to build an impressive prototype or version 1, which you can then use to get further funding.
Currently it doesn't seem to be that low ( "we invest a small amount of money ($14-20k + an $80k note") from http://ycombinator.com/ , but yeah, it's not a huge amount of cash. It is very early stage startups though, remember.
According to http://techcrunch.com/2013/10/25/y-combinator-13-7b-valuatio...: - YC company valuations total $13.7 Billion - 511 companies total
$11,000 + $3000 * # of founders = total investment in YC companies (let's just go with 17k for 2 founders on average) = $17K
$17000 * 511 = $8.7M
$100M - $8.7M = $91.3M
Clearly a pretty good investment =P