Bus to TCAT and then taxi is also very do-able.
I would guess approx. same total cost as N'EX + Taxi.

Advantages:
Bus allows you to avoid stairs (but there are elevators and escalators at NRT and Tokyo station). And the bus departures to TCAT are very frequent.

Disadvantages:
The bus trip takes longer than N'EX - especially if you hit traffic
(but the high frequency of buses can make up for that).
I don't like sitting on the bus - feel more comfortable in the train. (Yes, even in the crappy Bombardier coaches that they use on the N'EX these days - the original cars were so much better...)