A sportsbook does not publish a return to player. It prices its margin into the odds, and the margin is recoverable from the odds themselves.
Convert each price to an implied probability by dividing one by the decimal odds, then add them up. A two-way match winner market priced at 1.90 and 1.90 gives 0.5263 and 0.5263, which sums to 1.0526. The excess over 1 is the book's margin, and the cost per stake is about 5%. Priced fairly with no margin, both sides would be 2.00.
That 5% is worth holding next to the casino floor. European roulette charges 2.7% of every stake, the American wheel 5.26%, baccarat 1.06% on banker, craps 1.41% on the pass line, blackjack with correct basic strategy around 0.5%, and the better crypto originals 1% at 99% RTP. A wide esports market can therefore cost more per stake than the roulette table in the next tab. The difference is that the casino edge is fixed and published, while the book's margin varies by market, by tier and by how confident the trader is.
The arithmetic works on any market. Sum the implied probabilities across every outcome, subtract 1, and that is your overround. Compare the same match across two books and the cheaper one is visible immediately. Doing this for thirty seconds before a bet is the single most useful habit in sports betting, and it needs no model of the game.
One caveat on skill. Esports outcomes are influenced by knowledge in a way that a roulette wheel is not, so a bettor with genuine information may beat a specific price. That is different from overcoming a margin on average, which stays where the book set it.