The value of a bitcoin fluctuates minute by minute
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Who or what determines the daily value of a bitcoin?
When you buy from a retailer, they dictate the price that you pay. You can choose to buy at that price or you can leave without buying. There is generally no haggling on price though.
Market exchanges like Mt. Gox don't set the price. The price quoted comes entirely from a form of haggling by their customers.
Here's a video that explains how markets work:
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http://www.dailymotion.com/video/x871h9_real-world-economics-simple-supply_school (15 minutes - it includes a little propaganda at the end, sorry, but it was about the first decent video I could find that describes market price.)
So at Mt. Gox, customers put in buy orders and sell orders. Each buy order indicates a quantity and price that the buyer is willing to buy at. Each sell order indicates a quantity and price that the seller is willing to sell at.
The point where the buying and selling meets without a sale happening is known as the "cross price" (or sometimes called the "spot market price"). In economics this is called the "market equilibrium":
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http://en.wikipedia.org/wiki/Economic_equilibriumA market exchange like Mt. Gox doesn't care what the market equilibirium price is because they get a commission each time coins are bought or sold.
Other exchanges simply buy coins elsewhere and market them directly to customers at a given price. Their price generally includes a markup on the price that they pay. So the price will fluctuate because the price that they pay their supplier (e.g., when they buy from Mt. Gox) will fluctuate.
Now the reason the price at Mt. Gox's BTC/USD is always changing minute to minute is because there are always things going on causing people to want to buy or sell bitcoins. Thus the market equilibrium moves based on market conditions. This is not unique to Mt. Gox, nor to bitcoin exchanges. This is how nearly all financial markets operate.
The value of a bitcoin fluctuates minute by minute, and isn't the same on different platforms.
In order to sell your coins at Mt. Gox, you need to first deposit them with Mt. Gox. So your funds can only be at one market exchange at a time. So even though you might get a better price selling them at another exchange, for you to do that you would need to transfer them -- something that has a cost (in your time, at a minimum). So markets aren't perfectly efficient as a result -- there will be a buyers who find they need to pay more at one market or sellers finding they need to sell for less when using a different market. This too is not unique to Bitcoin or even to financial markets. The price you see for a gallon of milk is different from one store to the next. That too is an example of a market inefficiency.
Now there are traders who can make a profit by lessening these inefficiencies. They do this by keeping funds on several exchanges at once allowing them to buy where the selling prices are cheap and sell where the buying prices are expensive. The trading they are doing is called arbitrage. The price differences between exchanges provide arbitrage opportunities. There is a lot of that occurring, and there are sites that help provide information on these price differences:
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http://nyse-group.de/bitcoin-arbitrage -
http://bitcoin-analytics.com/#arbitrageif you are only buying small amounts, not much of this matters. But those doing large amounts of trading and wanting the best prices don't need to worry about going around to each exchange and trading at each -- simply trade at one exchange and let the arbitrageurs add liquidity. These arbitrageurs do profit from it but probably less than it would cost trying to spread your trading across a number of exchanges yourself.
Theoretically, the different values of bitcoin (in USD) on different platforms/exchanges is an exploit waiting to happen.
Except that performing that "exploit" is helping the markets by keeping prices in synch -- so we can know what price a bitcoin will fetch "at market",
So that's how various markets work.
