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).