There are a lot of people who assume that cameras only have some differences, but actually, there are some vital differences that will allow you to choose one over the other. Nikon D5300 is cheaper as compared to Nikon D800. The specs are very different. For example, the megapixels of the lenses are highly different from each other.
Nikon D5300 will come with a 24-megapixel camera while the Nikon D800 will have a 36-megapixel camera. Another difference is when they were launched. The Nikon D5300 was released back in 2014, while the Nikon D800 was released two years earlier. Even their sensors are different. Right now, the Nikon D800 is more expensive, mainly because the parts are harder to find.