I'll disagree with some points - mainly:

You don't need an exchange - that helps trading contracts, but OTC futures are bilateral.
Usually futures require an initial margin - an upfront payment, and margin calls to manage settlement risk.
Many cases you can be prepared to pay >$x if the price is $x now - reasons include time use of money/cost of funds, or even costs of storage.

As for the poor farmer example I agree - it can work against you too.  Funding cost of production ($3) in the hope of an $8 payoff crashes and burns when the drought wipes out the crop.  In order to satisfy the contract they might need to by at $20!  (based off a real example - farmers on wheat contracts walked off their land and simply defaulted.)