Edmund Edgar here, I run Reality Keys.
If people are doing a lot of transactions based on this data then it's not necessarily going to be huge compared to the data generated in the actual transactions. If you're using the pattern where you send data to a contract then people read it from there, that'll take some storage, but it can probably be freed up later once contracts that use it have settled.
However, sending large quantities of data to the blockchain and having somebody read it from there isn't the only way to do it. The way Reality Keys work is that we publish data off-chain, with a signature, and anyone who wants to use it in their contract can grab the signed data from our website and send it to their contract, which can check the signature then discard the data as soon as it's used it.
You can take this to another level with various economic hacks; For example, if you know that your counterparty can ultimately get the data from us if they need to and extract the payment whether you like it or not, you can set up an incentive mechanism - either a bond or just reputation - so that in most cases, you don't actually need to get the data from us at all: The counterparties recognize when they owe something to the other and just send each other money.
This is also the thinking behind state channels (in the bitcoin world the famous one is the proposed Lightning Network) that you'll probably want to use for fast trading at high volumes: By making it possible to enforce the settlement of a debt on the blockchain, you remove the need to actually do it.