Are the better odds at exchanges offset by the commission and premium charges incurred though? Reading the T&C's with Betfair the emphasis seems very much on 'the longer you go on winning the more we charge you'. Understandable from their point of view but no doubt you get clobbered as a long term winner.
sorry for the wall of text:
with betfair It's 5% base commission(winning bets only) and the vast majority of the time that still works out much better than the odds you get at bookmakers, it's a bit different for mma due to the lower liquidity so books are needed too. but often you can get bet's matched higher than books if you put an order up dependent on the liquidity and odds that you and others are offering/taking.
you will lower the commission you pay the more you bet also, by accumulating betfair points which is a tiered ladder, when you hit enough points you get a 2% discount on the commission paid each level, up to 60% at which point your paying 2% commission.
there's a thing called the premium charge which you refereed too, it's fairly complicated and I don't know much about it tbh. but they basically take a large part of any winnings, luckily you need to be making something like 250k a year to get it which most people don't. and even then this can be avoided by using other exchanges( betdaq,smarkets or matchbook) or books if you can get good odds to lower the profit you make on betfair, and avoid the charge. your better off finding some youtube videos explaining it though tbh.
matchbook are a great alternative for mma as
@Sham5916910 knows, they have good liquidity for it and the commission is a lot lower from the get go. i think it's 1.5% for bets that you take and 0.75% for bets that you offer and get taken. for most sports however the liquidity simply can't compare to betfair or betdaq a lot of the time. even smarkets (1% base commission iirc) have more liquidity for the most popular few sports which is understandable as their place as an outlet for matched bettors is pretty solid.
so when you look at the commission each exchange charges you and compare that to the average bookmakers overround which is usually 10-20% exchanges are much better in the long run generally.