Inserting an adsense after the first post in a thread has worked well on a couple of communities I've been involved in.
Try adding this to the bottom of postbit:
<if condition="$post[postcount] == 1">
<div id="postpromo"> <!-- you'll probably want to style this div - set width, centre and so on -->
your adsense code
</div>
</if>
As for revenue, it will be as varied as any other adsense campaign - dependent on traffic, keyword triggers, click through rates and so on.
As with any monetisation of sites, test thoroughly and try alternative positions, colours and so on. Perhaps consider serving adverts only to guests at first to gauge how successful it might be.