question you have to ask is do you really need flex...its a rather expensive excercise...
i just set my Adaptronic up with dual map switching...just a switch on dash flicks between tunes..the only luxury I don't have is that I have to fill up when the fuel tank is empty...but by emtpy it doesnt have to be drained, as long as its nearly empty and you fill it to the top with new fuel, if it still has 5-10litres of another fuel I think its hardly going to destroy your engine, especially if you have an afr gauge to keep an eye on it and it is tuned within safe parameters .
now In theory with my 2 tunes i can just install a flex sensor and the ecu will work out the rest by reading the sensor...how well this actually works in practice though I am unsure as yet..so how many extra costs are incured tuning wise i dont know, A flex sensor is about $250-300...maybe a tuner can tell us how much tuning is needed after that.
anyway my whole setup including AIT sensor, wideband, ecu and tuning came in well under 2.5k and I made half of that just by selling my pfc and Z32 afm, if i had an EBC i could of sold that too