Companies do have health and safety groups out there working on these things. Of course, the bigger the company the slower, more inefficient, and incompetent they tend to be. Prices can range - some things can be patched, but many times the reason for the leak is age and it requires a new piece of equipment. So, there you would have permitting to handle, expensing the equipment at either the field level or, most likely, well level, which would kill the "economic-ness" of said well/well pad which never sits well with a company and may make them rethink it and just plug the wells - equipment could be low to high hundreds of thousands - really depends on the piece and size. In terms of frequency, People are fixing these things daily, small projects to fieldwide initiatives. In terms of someplace like Turkmenistan, anything can be fixed with money. In that part of the world, your best bet is financial incentive. I mean, it is unethical, but just pay the "expediting fee" for the local warlord/mayor/president - usually a "donation" to an orphanage or fund that doesn't really exist. The cancer rates thing is interesting - the well in question is possibly a Marathon well. Their record isn't the greatest (Look into Paw Creek, NC and Marathon's "small leak" at their terminals there). No scientific data on this at hand, but as a person who works in oil and gas and chemicals with an oddly high number of friends who have had some type of severe cancers, I would say this would be hugely helpful to the general public.