Choosing the right commission structure is difficult and simplicity is definitely a factor, a flat rate of say 3% and is probably very fair, but if you are going to introduce a variable rate you should look at what you are trying to encourage, trades that add liquidity to the exchange are worth far more than trades that take liquidity off the site so maybe you could reflect this in your commission structure, maybe changing the structure above to only count for stakes that were put up as a offer to be taken by someone else or by applying a weight to offered bets, i.e. a offered bet generates X * the commission points of a taken bet

Other exchanges have attempted similar things, one was matchbooks old structure where if you put a price up there was 0% commission and if you took the price there was 1% commission on that bet, and there was no netting off for the market. Another is betdaq’s structure for (some?) api users, instead of the commission rate been decided by how much you match it is decided by what % of your bets are taken or offered, 100% offer bets gives you a 1% rate, a 100% take rate gives you a 5% commission rate etc

My biggest problem with the premium charge is that it gives nothing back to the recreational punter, if a exchange existed that had a premium charge as the only charge levied, effectively putting losers on 0% I think it could be very successful and could actually achieve betfairs original ambition of burying the bookies but it would probably not be very popular at the moment