I actually do the same, mixing 1.50's with 3's , but i play 9Man SNG's, and to be honest i ve thought about doing a BR analyzing the Average Buy=in, but it does offer some issues, like mentioned you can be beating the 1.50's but if u lose on the 3's you might actually end up a session losing, also by knowing this before hand, if you lose early on a 3$ it might affect your game play because you know best case scenario you ll end up that session ( that will still run for about an hour say ) breaking even which is horrible to look at, but as said i do the same, as winning a 3$ will give it a big winning session, so what i recommend is that open your tables, play your A Game in all of them and at all costs do not, under any circumstances, look at the table buy in, and that way you ll be playing your best and not according to buy in.