It all depends on where you live, what the market will suport in that area, and what your time is worth.
I live in an area with a very low cost of living, so my rates are still $40/hour $50/hour in a person's personal airplane, though I may raise them as I am busy. As others have mentioned, I charge in blocks of 3-4 hours for the most part. You may live in a high cost area where CFIs can demand and get more. Recently I had someone who wanted me to ferry an airplane- but balked at my daily rate. I told him I was sure he could find someone cheaper, but for me to spend 4 days away from my home and family, that was my rate. Otherwise it was not worth it.
Also, as an independent CFI you will want to get checked out in some airplanes. The Mooney (if you don't have time in it), is a great airplane but does have some different characteristics to it if you have not flown it. If you have not flown the avionics, autopilot... you'll need to at least read up on them which also adds time to your day. For example, the Mooney factory recommends stall training be done much higher than the 1500' in the PTS in case a spin develops. You'll also need to factor this time into what you charge.
Good luck, have fun and be safe.