"RIPA requires ISPs to log all internet sites accessed by their customers and to provide to the government on demand. This is a large potential cost burden on ISPs."
Sorry, your understanding is misinphormed.
I suggest you read RIPA again. The ISPs do not need DPI to log any of this data: they have been logging it for years. And making summaries of the data and selling it off to anyone who is interested in knowing which are the most popular sites, the most popular hosts, the most popular anything-else-you-want-to-know.
RIPA is there to protect the common citizen from having his data intercepted without a court order. Why else do you think the ad networks who approached the Home Office were so worried that their interceptions for profiling would be a breach of RIPA? The simple thing is that if the 2 parties to a communication give their consent to the interception, then that interception is outside RIPA. The ad networks and the profilers can show that consent because there is a cookie sitting on the browser and the site delivering the ads is hosting a script which delivers the targeted ads.
The thorn in the side for the profiler/ad network is when it intercepts a page which does not contain any script to indicate it is part of the ad delivery system - and that is 99.99% of the content on the web which is outside the ad network and being intercepted without permission, which means a breach under RIPA.
No one can use the DPI and profiling system to record where you have been. They keep telling us that they don't know who they are tracking; it is all anonymous because of the use of the unique ID they store in the cookie (like anyone who worked with removing the malicious adware and pop-ups believes that something called a unique ID is anonymous). Show me any tracking cookie that follows from site to site and try to prove it is truly anonymous.
Even BT tell us that they don't know who was profiled during the 2006/7 trials. They know how many customers where profiled but don't know who they were. (How can you count IP address connections when you don't know who is connected?)
The government has already paid the ISPs for the cost of the intercepts already done - they can't use DPI because then they would be intercepting everyone on that node, and that would be illegal.
The government really don't want ISPs to use DPI. As soon as DPI starts to be used, all the websites will become encrypted to protect their content from being used to create a profile of their visitors - that is confidential and highly sensitive commercial data that the profilers are trying to copy. Once content is encrypted, the government can't do its snooping without the delay and expense of breaking the encryption.
The only entity that wants to use DPI to make a profit is the ISP (and the ad network collecting the data for their profit). Quite why the ISPs want to annoy and alienate their customers by changing their role from passive data conduit to active data interceptor is something only the ISPs can answer.