As per the title. Some daytraders may have no choice but to trade the Asian session, perhaps because of work or convenience. So which pair is the "cheapest" to trade during Tokyo hours? I sourced volatility information from here and used transaction costs from my broker. Basically we're looking for the most volatility at cheapest cost.
It shouldn't come as a surprise to find that the JPY pairs tend to be the cheapest during the Asian session. The EURJPY costs more than the USDJPY, but you're getting more bang (or volatility) for your buck.
How does this compare with the London session?
As expected, the EURUSD is the cheapest pair to trade. We also get 35% more volatility per pip spent.
If you want to daytrade during the Asian session, it looks like the EURJPY is your best bet. However, this doesn't take into account your particular trading style or edge. The more-expensive pairs may be suitable if you possess a particular edge for those pairs (e.g. if you're skilled at fading ranges, perhaps the European pairs may be better than the JPY pairs).