Percentages and markup
At face value, buying ten percent of a buy-in entitles the backer to ten percent of what the player wins. Markup adds a premium: at 1.2 markup, that ten percent costs twelve percent of the buy-in, because the backer is buying into a player they believe has an edge.
Markup is only justified by a demonstrated edge, and a modest premium can consume all of a backer's expected profit. Backers should compute what the markup implies about required ROI before agreeing to it; players should be able to justify their number with results.