It'll take you at least an hour to get through Immigration/Customs and then the wait for the NEX and travel time will make it another 2 hours. Taxi to the Market means you might make it by noon...and as noted, most of the activity you want to see winds down around 7-8a and certainly all the workers will be headed home by noon. Though there is still enough to see in the mid-morning, you'll find nothing for all your efforts and cost. Certainly not worth the $100 you'd spend on the NEX and cabs. And even if you could get out of NRT by heliExpress, you wouldn't get to the Market until after 10am and spend even more to see very little.