Market price is market price, which is what people are willing to pay for it. You can pick any market out there (mtgox, bitstamp, etc). No different to the price of gold, if you wish.
Although an average price across multiple exchanges might be useful in a theoretical sense, the reality is that when you come to sell you will sell in a single exchange at a single price. Added to that the exchange rate of BTCxUSD is fluctuating so rapidly right now that attempting to use it to calculate what you might obtain even a week from now is probably not a sound basis on which to generate a financial calculation.
If you want to work out if mining for coins is worth it, then the general answer is "no". A more detailed answer can be found by using a mining calculator
Until now, it has has always been the case that purchasing bitcoins is a wiser way of spending your money than putting it in to hardware or even power to generate your own. Unless you're buying an ASIC-based miner I would suggest that this is the best way to approach bitcoins right now.