Supposedly purgatory's harsh conditions and Ban regeneration would make him to stay there for long, making him a lot stronger. What's ban power level when he gets out of purgatory?
Ban's power hasn't been calculated after his exit from Purgatory.
However, we can roughly estimate his power level. In the battle against Demon King!Meliodas, Ban manages to hold his own and fight him one-on-one for an extended period of time. On the other hand, Mael (whose power level is greater than 200 000, according to chapter 279) was effortlessly pushed back by Demon King!Meliodas in a previous assault.
We can thus conclude that post-Purgatory!Ban's power level goes far above 200 000.