The cost of a CMS will be completely dependant upon your needs, the more you want your CMS to do the more expensive the solution will be.
Yes, you can add a CMS to your existing site in some instances. In most however your site will probably have to be rebuilt to work in conjunction with the CMS & your content would have to be inserted into the database for use by the CMS.
Depending upon which CMS you choose this could be a yes or a no.
Some CMS’s have to have a specific layout & page structure to function. Most however do not & a CMS that hinders your design aspirations should never really be considered as it does not provide a “future proof” solution.
A good CMS will allow development within itself, allowing for existing functionality to be introduced into the CMS environment.
At this point in time; any CMS which does not allow your site to adhere to web standards should not be considered. It is now a legal requirement for any website which provides a service to be accessible, as stated by the “Disabilities Discrimination Act”.
A CMS should not effect your Google ranking or interfere in any other way with your Search Engine performance.