This would be very valuable for us, because as far as I know there is essentially no systematic way to locate these. We have a product which is meant to be of use to many niche audiences, making them difficult to find. We are very early stage, and are just now getting some initial partners to work with and looking for ways to market the service to initial customers (in the thousands).
I would most prefer a marketplace where I would know up front how much it would cost to sponsor a newsletter. Without that, my concern would be friction in dealing with lots of small organizations directly, though we would still be willing to do this at our stage. If payments are happening through the site perhaps you would just take a cut, but alternately if this worked reasonably well and had a fair amount of newsletters for us to find, I would probably pay $10-15/month to use it.
I'm a little fuzzy on the mechanics. If you focus on a couple quality offerings every week, won't those particular newsletters get inundated with requests?
Also, how do you plan on generating revenue?
v4 could then offer manually created ad units, and v5 could be self serve.
The project was previously featured under the name Mailman on HN [0].
Of course if the feature of showing metrics (subscribers, open rates etc) proves to be popular enough they might consider ways to rank newsletters using metrics that are publicly verifiable or internal metrics from running Upstart after several months (that cannot be easily be gamed).
Hopefully @pixelfeeder will respond.
Then there's contacting the newsletter directly for proof of their actual numbers by making available current and previous month's screen shots of MailChimp campaign reports, for instance.
And as pixelfeeder mentioned, they'll also gather their own internal analytics based on how money changes hands on Upstart.