My wife and I just returned from a backpacking trip using a small 2 person, no longer made, REI tent (Coupe?). It has a lot of mosquito netting so I have an all fabric cozy that covers the mosquito netting.
The outside temps were 33F and 35F on two successive nights. Inside temps didn't get below 50+F while we were sleeping on either of these nights. So, assuming some thermometer variance from my cheap thermometers I'd say the tent was about 15 degrees warmer than the outside air. Even assuming a lot of thermometer error would give us at least a 10 degree bump.
The night was dead still. No wind. While sleeping, a 10+ degree increase in temps inside a small,all fabric, double wall tent is typical for me. A temporary increase of 15-25 degrees is also typical while active within the tent (e.g. while changing clothes).