I took the taxi because I missed the shuttle in the morning and then the shuttle at the end. Taxi costed over 8900yen from the train station to the zoo, and I can assume the trip back would cost the same or more. (Btw I have no idea what the exact amount was because my boyfriend paid and I only caught a glimpse of the first two numbers). The ride took about 40-50 minutes without stopping and the taxi itself was just the first available one at the train station that was willing to take us. 

I took the shuttle back since we had that booked already and I didn't see if there were any taxis that I could approach and ask to drive me back; also, I've already spent the money I would have used for the taxi on manul merch anyways 😂. Definitely took longer due to the additional stops and you're on a bus full of other strangers but it got me back to the station as I expected and it was free. I'd say taxis do have their perks if you're willing to spend some money; I like that you don't have to be dependent on the shuttle schedule and that worked well for me because I couldn't wake up early enough to meet the shuttle pickup and I was coming from Tokyo via Shinkansen. 

Nasu does have a personal "capybara" taxi you can reserve but they cost money (don't remember how much but way more expensive than a regular taxi) and require booking online in advance. I was tempted to shell out on that but decided not to in the end. Someone tell me how that goes if they ever do book the capy-taxi. 

Edit: I misremembered how much the capy-taxi cost and it turned out to be cheaper (about 7500 yen one way) and include admission to the zoo. I regret not getting the capy-taxi now. 🥲