If using Bitcoin core client am I constantly going to be downloading data from the blockchain? I understand you have to download the entire blockchain and sync in the beginning but don't you also have to constantly keep the software on if you want to buy/sell stuff, which means more data being downloaded to my personal comp every instant there is a transaction. 

If this is true, what is the average amount of data per month being downloaded so that I can use bitcoin. 

Also if this is true, why isn't this point highlighted on bitcoin.org, they say you have to initially download the blockchain but not that its constantly going to be downloading data on your comp, data is expensive!
At the time of writing, the blockchain is 24 GB in size after 5 years. This is an expansion of 0.12 kilobytes per second. However, this does not represent all of the bandwidth used by the Bitcoin client: 


The Bitcoin client also relays transactions and blocks it downloads.
The protocol has overhead.
Sometimes the Bitcoin client will download transactions that don't get into the blockchain.


I tested the amount of bandwidth that Bitcoin uses by measuring it using bwm-ng, over the course of six and a half hours. I found that it uses 2.1 kilobytes per second, combining upload and download. Over a month, that would add up to 5.5 GB.

Note that my Bitcoin client did not accept incoming connections, which would have changed the result significantly.

This can be an expensive amount of bandwidth. In that circumstance, you can use an alternative, like BlockChain.info, or MultiBit, both of which use almost no bandwidth.