For a second I thought it was pay per post, only to find out that those were weekly payments. I feel pity for participants in your campaign, be sure that they won't spam all over the forum since moderators might not like it, consider this as a friendly advice. I won't help in promoting your campaign but you better review your rates for the sake of all the participants who will be participating in this campaign, or only a few participants will be interested in promoting your campaign.
I think I can be considered as the 'godfather' of Micro campaigns lol cuz I started this. But here's the deal, it is not us managers who are deciding the rates, its the project owners.
This is essentially advertising, not a job, so no pity required. The rates are what the project owners can afford to spend. If they have a large budget, they'll get more participants, hence more exposure, else, they'll get low quality posters. However, not everyone who applies in these Micro Campaigns are low quality posters. And as a campaign manager it is entirely plausible to not allow the shit posters into such micro camps.