Let's say it's a long-term partnership. Now it's only $21 but when Bitcoin returns to its previous rate, rent will cost $40-50 per week.
In my opinion 1 mBTC is very suitable for renting an avatar, especially if it is in the long-term, firstly bitcoin is on the rise and secondly wearing an avatar does not cost any extra effort. This is an acceptable additional income especially if the member is not participating in a campaign.
But this varies from person to person, some may say that this amount is low and their avatar is worth more than that. In addition, some campaigns require the use of avatars.