SDDs are at a price point to space ratio where if you can afford a mid-high to high tier graphics card, there's almost no reason to not be able afford a SSD. They're also frequently on sale so it's not hard to find a deal to be had.

Game performance wise you will get better level and model loading speeds. FPS isn't really effected unless the models/map textures being used are high res but generally games have these things cached in the memory before seeking to the storage location.