The value for TBT - IGB exchanges depends, it changes a lot. The best way to know about recent rates is just to go to recent exchange threads in the TBT area and see what they've been going for.
If you're asking about villagers, the List of Villagers by Popularity (in the Villager Trading Plaza) has the average price for each tier but I don't know how updated that is. Just search for villagers in that tier being sold or auctioned and see for yourself.