By time seen I don't literally mean time displayed on screen but more like TV/radio ads, as in this ad will be part of our rotation of X ads for an entire months across X publishers. I think The Deck used to do this.

Determining the price will be a bit tricky (and I would expect that you'd have to lowball yourself until your platform builds credibility in terms of good ROI) but in the long run it should mean your advertisers pay a flat price to be included per week/month regardless of actual impressions or clicks (thus there's no fraud potential as only the raw profit from the ads will matter - the only "fraud" potential would be to literally buy the advertised product en masse).

Gotcha, that definitely makes sense. We are looking at doing that for some of our larger sites, similar to Daring Fireball: https://daringfireball.net/feeds/sponsors/ -- which I believe is based off the old Deck model :)

Thanks for following up.