Why CPM is the wrong model at small scale
CPM prices impressions, and impressions are the metric you can least honestly measure on a small static site. If pages are cached at the edge or served as static files, a page view never reaches your code, and counting one means either estimating or instrumenting every reader with a script.
Worse, impressions are the metric advertisers trust least, precisely because they are the easiest to inflate. At small scale a click is both more honest and more valuable: it is a real person choosing to visit.
Setting a floor with no history
You cannot derive a price from data you do not have, so do not pretend to. Set a floor low enough that the first advertiser is taking a small, obviously-survivable bet, label it explicitly as a placeholder, and say that it will be repriced once there is real click data.
Doing this in public is a feature. An advertiser who can see that your floor is provisional and your click counter is real will trust the second number more, which is the one that matters.
The structural decision: ranking or draw
If you sell a fixed top position to the highest bidder, readers learn to skip it and you have sold your most valuable slot into worthlessness. A bid-weighted draw keeps the page worth reading while still rewarding a bigger budget. Pick the mechanic before you pick the price, because it changes what the price is for.
Keep paid placement out of your editorial rankings entirely. Category pages and "best of" lists that can be bought stop being worth reading, and that is the asset you were monetising.
The payments problem nobody warns you about
Before you build a checkout, read your payment provider's acceptable use policy for the word "advertising". The merchant-of-record platforms aimed at software — Polar and Paddle among them — explicitly prohibit payments connected to advertising and sponsorship and list directories as restricted. Stripe permits the category but is not available in every country.
The failure mode is brutal: you onboard, take advertiser money, and the account review freezes the payout. Invoicing by hand for the first handful of advertisers costs you an afternoon a month and avoids the whole category of problem.