Car rental: will renting an ad hoc car in New York City be much more expensive than reserving it in advance?

The prices and availability of rental cars will vary dramatically according to demand. If demand is high for a specific period, then as you get closer to that time, prices will go up (sometimes significantly!), And individual locations may run out. If demand is low, prices may fall as the date approaches.

In general, the best option for someone in your situation is to make a reservation that allows you to cancel without paying a fee, and then monitor the prices as the time approaches. If the prices go down, you can cancel your existing reservation and make a new one. If it turns out you do not need the car, you can cancel the reservation without charge.

Since you are not from the USA UU., If you end up making a reservation while physically in the US. UU., You must make sure that your residence is correctly selected in the site from which you make the reservation. If you do not, then the necessary insurance will not be included in your reservation, and you must pay them at the time of collection, which will significantly increase the price.