So even if fuel is inexpensive now, it can become expensive quite quickly.
What we see as consumers is conflated by the fact that there are self-imposed limits on extraction by OPEC designed to raise prices. Due to various political situations, those limits have been slowly removed. When you see predictions of "Peak Oil", often they are based on existing production and not maximum production (and hence are nonsense).
I'm not aware of any literature that attempts to speculate on current maximum production capacity. Probably it exists, but I certainly can't find it. Whether we have hit "Peak Oil" already, or whether it will come in the future, I don't know. A lot of that kind of stuff is politically very sensitive and oil companies/OPEC are understandably reluctant to be straight forward with the data.
Finite resources eventually run out. When you don't know how much you have, it's tempting to assume that because you are finding more, you will continue to find more. There is no real guarantee of that. It's a risky strategy.
Decommissioning on the other hand is remarkably expensive because of secondary nuclear waste like contaminated reactor vessels and concrete make up 99% of the nuclear waste.
[1] http://www.world-nuclear.org/information-library/nuclear-fue...