It is a tossup between wildlife and sports. Both need long lens, highly performant sensors, great auto focus and reasonable speed. Wildlife often needs the longest lenses, sports need the fastest bodies. Indoor sports can be very tough because you need to maintain high shutter speeds in dim light with long lenses - territory of the 200mm f/2.0 and Nikon D4s or Canon 1DX bodies.
landscape can be very cheap because typically the lenses are stopped down a lot do cheaper lenses can be sufficient, you don't really care about speed, high ISO, focussing. However, at the high end it can be expensive. You often want the best possible sensor and detail, this may push you to medium format cameras, or at least a high end DSLR with great glass, TS lenses etc.
There are other costs. E.g. professional wildlife and landscape photographers typically need to travel to their subject, and might have to send months in places like Patagonia, Greenland, Borneo to get the desired shots. For rare animals the pros for national geographic can spend years and years of trekking through the Himalayas to find suitable subject and conditions.
Conversely street is easy if you live in or near a big city. Wedding work you get people asking to be photographed and they even pay you to do it, while a wildlife pro will have to fund their own trips and then try to sell the photos afterwards (unless they are very lucky NG togs etc.).
or sports,my goodness £90,000 pounds for a lens 
