I've heard many times the claim that drug companies purposely make treatments and avoid selling/researching cures instead. Are there any documented cases of this behavior happening or any concrete reason to believe this is the case?
I apologize if this sounds a little rude, but the question does not really make much sense.
There is no possibility to "avoid cures", for many reasons. One being that the drug development pipeline takes too long and is too inefficient to let it go when finally a good product is obtained (see graph).
Most of the research funding comes from Government agencies, not pharmaceutical companies. The basic research to find the mechanisms of disease and thus allowing the developing of new drugs are done in Universities/Research Institutes. Big pharma companies may run (usually in cooperation with universities) large-scale chemical compounds testing and fund some pre-clinical studies and the clinical trials (again, usually in collaboration with universities/hospitals).
It is plausible that a company may delay the release of a new drug to maximize the revenue of an older generation drug, but it is impossible for them to block the research in other places. So, if they have a magic cure, they will try to sell it as fast as they can. If not, the drug may be found by some other company or even may be published by an academic institution, preventing the use of any patent.
US Pharma Industry R&D spending anual budget:
The difference stems mostly on research performed outside the US and post-market follow up of drugs. Source: Congressional Budget Office 2006.
Of this budget, Canadian data suggest that approximately 15-20% is spent on basic research (i.e., drug discovery), 50-60% in preclinical and clinical trials and 20% in bioavailability and post-market (phase IV) studies. Source: Canadian Patented Medicine Prices Review Board Annual Report 2009. US-based pharma companies may spend close to 80% in clinical research. Source: Applied Clinical Trials.
Most importantly, many of the drugs that have appeared in the market derive from Government-funded research. In the last two decades, universities and research institutes have been patenting its own research and have reached licensig agreements with the pharma industry. There is a very good article about this issue: Stevens et al, N Engl J Med 2011; 364:535-541
US Government annual budget:*
.* It does not include State budgets, Senator's discretional budgets or smaller program budgets scattered in diverse departments.