That isn't how bitcoin works. Bitcoin can't spend part of an OUTPUT. So all tx must include entire OUTPUT(s) not entire wallets. So if you have 100 BTC in your wallet, you want to send 10 BTC, and the smallest output (or sum of outputs) which will result in no fee is 12 BTC then you send 12 (100), 10 to the destination and 2 back to a change address.
Still what makes calculating tx value difficulty is that it is never possible to know for sure which is the "payment". In the example above you could be sending 10 (w/2 change) or sending 2 (w/ 10 change).
BlockChain.info attempts to make educated guesses to determine which was the intended tx.
Raw Volume (sum of outputs)http://blockchain.info/charts/output-volume
Guesstimate on like tx volume (based on size and "roundness" of outputs tries to determine likely change and removes it from daily sum)http://blockchain.info/charts/estimated-transaction-volume
The estimated tx volume is pretty steady at 100K to 200K per day.