A patent gives you the right to stop others from practicing your claimed invention; it doesn’t give you the right to practice the claimed invention.  That is entirely counter-intuitive, but that’s the way it works.  It is entirely possible for a device or method to be claimed by two different companies or inventors, and neither of them can freely practice the invention.

Usually about $10,000 for a formal opinion.  But we have issued opinions for as low as $5,000 and over $20,000.  Informal guidance with respect to a single opposing patent can be as little as a few hundred dollars.

No.  There is absolutely no requirement to secure a right to practice opinion, or even a right to practice analysis, prior to filing a patent application.  Indeed, it is relatively rare for an inventor to do so.

No, but it may well be a good idea to do so.  Aside from incurring damages and legal fees, it is very frustrating to spend a year or more developing and getting a product or service on the market, and then have to shut down operations in the face of a competitor's patent.  Depending on market practices, it might also be necessary to buy back infringing product from one's customers.  Note that as a general rule there is no compulsory licensing in the United States.  A competitor holding a patent has not obligation whatsoever to license his patent to you.

Yes and no.  One can purchase defensive patent insurance, but the insurer will usually require a right to practice opinion to identify the likely problems, and will try to exclude such problems from coverage.  In addition, although insurance may cover some of the legal bills and damages, such policies almost always have a very significant co-insurance (usually 20%) and a large deductable (often a million dollars or more).  A typical defensive patent insurance policy can easily cost $100,000 a year.