Every time I see a calculation of the profitability of mining taking in account your hardware and the number of Hash/s it does and subtracting energy costs and then the probability of finding a share and receiving 25BTC every block and so on... it never accounts for the money given in transaction fees (that will eventually be the only money given to miners when the stock of BTC reaches 21 million). I'd like to make a more accurate estimate of how much money for MHash is actually being generated that takes the transaction fees into account. I can't find that info on Block Explorer. Does anyone have an idea of the magnitude of those fees?
There are some useful charts that show information about, amongst other things, fees per block. Fees per block are currently trending downwards, but that is due to the additional hashing power coming on-line which means that blocks are being mined faster and so the average number of transactions (and hence fees) in an individual block is dropping. Expect this to balance out sometime soon as the network takes account of the increased hashing power out there.
Right now, if you assume that transaction fees sit at around 0.25BTC per block you won't go far wrong.
Currently the transaction fees are considered negligible compared to the block reward.
On this page you can see blocks as they are mined. The last block until now f.e. has 0.0258 BTC fees for 26 transactions. Blocks with more transactions, like this one f.e., earn 0.1796012 BTC for 211 transactions.
As you can see, these are really not much compared to the 25 BTC block reward. But as the block reward halves every 4 years, there will come a time in which transaction fees will matter very much and in which they will be the driving force for miners.