What is the average temperature drop per thousand feet of elevation?

In the lower atmospheric regions (up to altitudes of approximately 40,000 feet or 12 km), temperature decreases with altitude at a pretty uniform rate. Because the atmosphere is warmed by conduction from Earth's surface, this lapse or reduction in temperature is normal with increasing distance.

Although the actual atmospheric lapse rate varies, under normal atmospheric conditions the average atmospheric lapse rate results in a temperature decrease of 3.5°F/1,000 ft (6.4°C/km) of altitude.
https://en.wikipedia.org/wiki/Lapse_rate

The temperature of the atmosphere actually heats up when you get above the dense gas altitudes. Above the Ionosphere the temps of molecules can vary between 2,000 and 3,000 degrees C.
And even in the lower atmos[here there is a thermal inversion zone where temperatures can go from 0 degrees C to around 35 degrees C in a matter of a few hundred meters. Our atmosphere just doesn't keep getting colder the higher you go.

https://en.wikipedia.org/wiki/Thermosphere