We've been "doing" the loop since Oct 2012, when we repositioned from Orange Beach to Tarpon Springs. In 2013 we traveled to Norfolk and spent the summer of 2013 exploring Chesapeake Bay. Last year we traveled from Norfolk to Lake Barkley, KY. This year we have done the Cumberland River and will do the Tennessee in the Fall. Don't know when we will "finish" the loop, right now we like the inland rivers.
How much does it cost to "do" the loop? It depends on how you do it, i.e., marina's vs anchoring out; dining out vs dining in, etc. We found our habits didn't change much, didn't eat out more often than we normally do, eat most meals on the boat; didn't have slip fees and anchor out most of time, so cost of marinas was similar to normal slip fees when not cruising, etc.
The big variables are fuel and boat repair. Fuel can be planned using known burn rates and distance of about 6,000 miles. Boat repair is unexpected and consists of mechanical breakdowns of engines/systems, bent props and damaged hulls, etc. Our largest single expenditure (aside from fuel) has been $1000 deductible for fiberglass repair. We know folks who have had engine and transmission rebuilds, trashed 1 or more pairs of props, lost refrigeration, etc.
There are people doing the loop on a shoestring and those spending multiple thousands of dollars a month. The experience is the same for everyone, regardless of the amount spent.