it's not "up to". for a 250W MH/HPS bulb, you need a 250W ballast. you're looking at between $100-150 retail, look around at ebay and such. you will often get what you pay for as electronic ballasts are silent, while magnetic often make a noise that has the strange side-effect of making you want to pull your hair out.
while you can get ballasts that support both MH and HPS, most go the route of conversion bulbs. you can then buy a HPS ballast, and use a special MH bulb designed to operate with the same wattage HPS ballast for veg.