Just recently my car started to make a noise as if something is dragging on the ground but it really isn't. It only kmakes the noise when the car is in overdrive. I know the tranny slips a tiny bit but only if you floor it. I checked the tranny fluid and its looks dirty but i wouldn't say it'* filthy. Is my tranny toast. If so what is the cost for basic rebuild

First thing, install new oil after you drain and change the filter.

Sorry, we can't tell you from a noise you describe that your trans is good, bad or anything. For that you will need to bring the car to a transmission specialist and have it diagnosed properly with scan tool etc.

If your fluid is dirty, a filter and fluid change are in order as Mike pointed out. Personally, with that noise, it would be best to ask a shop for their opinion.