A big old question! Nuclear plants generate different amounts of power. This is down to how efficient they are, how large the reactors are, and the type of fuel. In the UK our oldest civil nuclear reactor, Calder Hall, only generated 46 MW of electricity (only 23% efficiency) but Hinckley point C which is currently being built will generate about 3200MW of electricity!
Similarly the costs have increased over the years as systems are both bigger and contain more and more systems and structures that are designed to make power stations safe and secure. A power plant in the current day could be expected to cost over 22 BILLION pounds! Then running and maintaining and finally decommissioning is a whole lot on top of that. Maintenance I’m not sure about but I know U02 (Uranium dioxide) which is used as the fuel costs around £1000 a kilo and in a nuclear reactor you’ll have about 1000 tonnes (1000000kg!). This is likely to be a small part of running costs!
The Nuclear Decommissioning Authority is the UK’s body in charge of decommissioning our old nuclear reactors and they spend about £2.4 billion a year on this work. Decommissioning activities are expected to be going on for at least 100 more years up at Sellafield, the main site for UK decommissioning.
Essentially…nuclear plants are VERY VERY expensive but it’s also key to remember that so are other energy generation plants and if you compare the amount of time a coal plant can run without refuelling (maybe a month or slightly more) with a nuclear reactor (two years!).