A simple Google search using "how much does a nuclear power plat cost to build" will answer that... In short, it is many billions of $$$ though a huge part of that is from time lost (in some cases decades) and legal expenses due to every 'environmental' group in any given country fighting them in court. In more than a few cases construction was drawn out for so long that it became necessary to finish building the power plants to use natural gas fired turbines instead of carbon-neutral reactors.

As for discounted tariff: Ja. As others have said, the mining farm is right next to the power plant so in a sense the farm is 'behind the meter' in that there is next to no transmission costs involved. Yes obviously the power plant is metering the farm, just separately from what is being sent out to 'The Grid'.

Now one might ask, "why would a utility give such a low rate to the farm"?.
Simple: Any power plant needs a stable base-line load to run at best efficiencies. Mining farms pulling 10's of MW are as stable of a load as it gets. Couple that with the power plant being in a region that is not heavily developed with heavy industries or cities near it (yet) and a rather long distance to The Grid's high tension lines it becomes simple economics with the Utility running the power plant making more money power the farm vs selling it on The Grid.

Being Nuclear and therefore immune to the fluctuating costs of natural gas/oil/coal the Utility is able to provide TerraWulf that sweet 5-year power contract. A rare Win-Win for all parties involved!