You are probably better off just offering the smallest house edge you can live with.
Changing the odds based on how often people bet probably won't work too well because that will encourage people to bet too much, which will probably make them go broke a lot faster than other ideas. You can shear a sheep many times, but you can only eat it once.
I also don't think varying it based on profit is a smart idea either. The profit should always be going up, a counter on the website showing exactly how much money you are winning (aka how much bettors are losing) doesn't seem like the best play. Also, if it updates in real time and the odds change after every bet, you are going to have more people walking away after wins. If I just won 1.99 on a 50% bet and it tells me 1 second later I can only win 1.97 on that bet I'm a lot more likely to stop.
Actually, I appreciate your pains. But try to look at this from a different angle. Say, you are the casino, and today you earned enough to call it a day (no pun), so you have a few options to reward your userbase. The most obvious is to just give away the money. But if you would distribute the surplus evenly to all players, no one will be quite happy, since people are envious. If you endow just the most "valuable" players (whatever you may think by that), others will most certainly feel disadvantaged and miserable (since people would be even more envious). Then you can lower the house edge, or even make it negative for some time, so you give the players more incentive to play further on. And everyone will be happy with you, right?
