On sponsors, I don't think there's always that much influence from sponsors, typically it's an exchange of money for booth presence, mentions during the conference, and some social media marketing, there was no control over content from them.
Costs are very geography specific. For example in the UK it was possible to put on a conference for 100-200 people in a decent venue for ~ £20-25k. This was not for profit obviously, but we kept ticket prices low and relied on sponsorship money to break even.
I'd agree that the first year is the hardest, we had to pay deposits out of personal cash while working on sponorships, so that's tricky unless you've got a source of funds. After a while it gets easier with sponsors as you're a known quantity and hopefully you have some cash from previous runs available for next year's deposits.
The most expensive items are anything that's per attendee. Even a small expense (like snacks or lunch) gets expensive once you multiply by 200!
The speaker contract thing was also odd to me, I've spoken at > 20 conferences and I've never been asked to sign a contract (also never been paid to speak but that's the reality of most tech conferences)