Nick,
Glad to see you pick up this thread. I have talked to 4 or 5 people on various Joomla sites who are looking for this, and at least that many again have similar comments in recent threads. One I emailed with said he'd be glad to pay $200. So would I. However, if you wanted to sell many, a typical market price for a module that handled Paypal IPN would be more in the $50 range.
I used to work with phpnuke and they have several vendors who sell a Paypal subscription module in that range. But phpnuke is plug ugly compared to Joomla and CB.
I have corresponded with the famous PixelBunyIP about this, and she is looking into it with interest. As I told her, I am a marketing pro and would be glad to promote this on various Joomla sites, although you are obviously well connected yourself.
If I can't get a Joomla-CB2 solution in the next few weeks, I may have to go with Mambocharge, which only works with CB1. It's scary to think of uninstalling CB2 and going backwards. Jeff at Mambocharge/mamboresources says his programmer "crapped out on him" instead of completing a version that works with CB2.
Hope this helps, Keep us posted,
Buck Lawrimore