The role of money is to facilitate trade. Some have argued that the value of Bitcoin is relative to the cost of mining; however, this approach does not take into account the function of money. As it stands today, Btc functions very little as a money, so calculations based on the cost of mining + speculation are understandable, and perhaps even correct. However, if Bitcoin does expand and becomes a method of payment, this calculation will be inherently flawed.
The "equation of exchange" is determined by the relationship between money supply, velocity of money, the price level and an index of expenditures. M x V = P x T, where M = money supply, V = velocity of money, P = average price level of goods, T = number of transactions. However, in the case of Btc, the cost of mining must be added to the equation. M x V = P x (T + Y). Y being the cost of mining, which counts as an additional cost to each transaction.
i.e. On a yearly basis services and goods of a value of $12M are traded through Btc. At that point of time, maybe 8M Btc exist, and maybe 500.000 Btc users exist. However, around 5M Btc are likely to be held by early adapters who do not use the Btc to trade goods and services with. So 3M Btc must facilitate trade. In this scenario, each Bitcoin must be transferred 4.3 times per year, however, Btc are likely to be transferred more than 4.3 times per year. However, suppose each Btc is only transferred 4.3 times per year. Say the cost of mining is $1M per year. A reasonable price for Btc would perhaps be between $5-10. EDIT
M x V = P x (T + Y) 3 x 4.3 = 13 (where 1 is the value of Y)
Based solely on the trade volume versus trade circulating Btc, price would be $3. If each Btc was transferred more times, the price of Btc would be lower. However, if the Btc economy had grown in 1 year to become a $12M economy with 500.000 Btc users/owners, potential for further growth would obviously exist, which could account for $2-7 of Btc price. If some early adopters were to cash out and their Btc were bought by traders rather than "hoarders", the price of Btc would fall in a rough proportion to the amount "released" into the economy. Speculation in Btc would reflect all the inherent price dependents, and speculators would keep track of the amount of Btc released into the economy, so although the average trader would not possess relevant market information, the price of Btc would more or less equal the value of Btc due to competitive speculation.
However, if the Btc economy were to have a value of $120 m per year and only 8M or maybe 8.5M Btc are in existence, of which perhaps 6M Btc facilitate trade (if the economy was that large and stable, I suspect many early adopters would in due course of time cash out on a fair amount of their holdings, which would not sink the price and which could be invested more profitable,) the price of Btc, without taking speculation into account, would be somewhat less than $20, depending on the amount of transfers of each Btc per year.
All of above numbers are just hapzard guesses, however, the above calculation could be used to forecast the price of Btc in the future, and also determine whether today's price reflects reality or not; or so I believe.
Disclaimer: I withdrew my USD from Mtgox without having made any trades, and am not looking to put any money into the market before an actual marketplace exists, or is at least far more developed than today.