Igloo price depends on:
1. Location. The closer to the tourist places the higher price. For example, glass igloo next to Santa Claus village in Rovaniemi might cost 700-850€/night in December while igloo near Levi or Saariselka resorts 170-250 km north from Rovaniemi will cost around 300-350€/night.
2. Comfort/luxury level and facilities. Glass igloo with basic facilities (at least wc and shower inside) will start from 200€/night. More comfortable options with kitchenette, own sauna and probably outdoor heated jacuzzi will cost 550€/night as minimum.
3. Season. December - March is a high season with the highest prices. Late August - end of November is the season of autumn Auroras and prices for igloos are a bit lower. Also in April it is possible to find good deals for igloo stay.
Keep in mind that you are paying for an experience, not just accommodation.