The responses on my Reddit thread should answer this question: http://www.reddit.com/r/BitcoinMarkets/comments/25dy7l/streaming_market_data_for_bitstamp_coinbase_btce/

It would be about $100 per month to support just you and ~$300-$600 for everyone.  PubNub gets pricey with this kind of message volume....I just started a conversation with them to negotiate a volume discount.  This is my first month not using a free demo account with them so I am having some difficulty nailing down a solid number for their costs.

The only additional cost for turning on data per exchange is the additional message costs. So I really need to figure out what their volume discount will be.