The money is coming from their very action, as I understand it, the blockchain allocates Steem every time you post/vote/comment

How much is determined by the algorithm put in place by the Steem Team...

As far as the funds so far, they have been generated by mining the Steem blockchain, the Steem is then fed back into the blockchain (as Steem Power).

So under the old rules, you were giving out some of your Steem Power each time you voted, the newly proposed algorithm, gives away Steem for voting and commenting, just more for commenting and more for voting on comments.

If the market cap for Steem doesn't change, and nobody outside of Steem trades in Steem Dollars, then the value of Steem slowly diminishes as there is a natural inflation rate, coded into Steem.

In this scenario, the money is just going round and round, slowly diminishing in value, like it is at the moment; hopefully, because of the way Steem Dollars are pegged to the US Dollar. There are loads of hungry traders out there, ready to trade in Steem Dollars.

So come July the 4th, when lots of us cash in our allowed amount of Steem Power, to Steem Dollars, there will be lots of Steem on the market.

Therefore, the hope is, that a sudden increase in supply, coupled with an increase in demand, will drive the price up, making our Steem Power worth more, and subsequently, the value of our posts more.

So the short answer to your question is; the money comes from people trading Steem on the exchanges, if they then put it into Steemit, the money is directly used to fund, posters and curators.

There is also a way of purchasing debt bonds from Steemit, but I don't really understand that, but I'm sure that generates funds as well.

Hope this helps

CG