If an igloo is unoccupied it will be about the same temperature inside as out.
If it's occupied, it's hard to say. It depends on how large it is, how many people are in it, how much clothing the people are wearing, how it was built (how thick the walls are, how well the cracks are sealed, if it has something to keep the warm air from going out the door), etc. But, I would make a wild guess that 10 to 30 degrees F warmer inside is typical.
But, if you add heat , say by cooking on a camping stove, then the temperature can really go up. Melting the inside of the igloo is not hard to do. I suspect temperatures 50 deg. F or more above the outside temperature are reasonable when you have a one burner camp stove running on high.