Where do the prices on a partner's own website come from?

The same prices set in the portal's Pricing tab — the ones that actually bill the restaurant. There is deliberately no second price list attached to the website, because two lists eventually disagree and the one on the website is the one a restaurant would hold you to. Change a price and the site shows the new one straight away. Set none and the site asks restaurants to message you rather than inventing a figure, and the portal warns you it is doing that.