If you want a gaming computer than it could range from $700 - $3000. $3000 is a bit high and you'll probably only get to that price if you want water cooling or liquid nitrogen cooling(if not higher). I would say that a good motherboard is going to range from $150 - $300, case is something personal so if you can tell us what you like in a case we can get you a better price range. Hard drives are not as pricey, but still with the case, decent motherboard and hard drive, you're already over your supposed price range. I would suggest looking around newegg.com, or tigerdirect.com, get a small theoretical build together, submit what you've found, then we will have a better way to get you in the right direction.
Well you are talking about making a new computer - something that runs programs, a web browser, something that you can upgrade. Much more than just something that you put a game disc into and play or use a mediocre web browser on. So yes it is pretty hard if not impossible to put together a gaming computer that has equal graphics of the PS3 for $300. If you get lucky that's what you'll find a motherboard and processor going for.