Cheap enough for a single medium-sized nation to be able to buy and build a reactor without crippling their economy or having to have an international whip-round
An extended period of time would mean the length of the reaction being longer than the time taken to cool down, clean and prepare the reactor for another ignition. At the moment, the reactions last tiny fractions of a second.
A production reactor would have to work for years at a time without requiring any internal maintenance.
Well, once the energy is released from the fusion reaction, the real aim of the game is to convert it to electricity. We can't do this yet
This is the point though - we can't.
A self-sustaining reaction is one where, once ignition is achieved, the reaction on its own produces enough energy to keep the reaction going. All one has to do at this point is feed the fuel in.
We can't do that even on a small scale yet - not even close
But even that isn't enough. The reactor would have to be able to power itself and its safety & control systems. That's a whole different pint of prawns - especially if electro-magnetic confinement is used.