It will always cost about the same to produce a bitcoin as the bitcoin is worth. If it costs more, people will stop doing it and the difficulty will drop. If it costs less, people will profit by doing it and the difficulty will rise as more people join in.
It's already difficult for individuals to produce bitcoins, but people have solved that by pooling their resources, getting regular rewards of fractional bitcoins.
That's helpful, thanks. Still, it's only profitable to mine bitcoins at any computing cost if they can be exchanged for other items of value. If the value of bitcoins drop and people leave the bitcoin economy, making the last N bitcoins easier to compute won't make them more valuable.
It's already difficult for individuals to produce bitcoins, but people have solved that by pooling their resources, getting regular rewards of fractional bitcoins.