Where downtown are you staying? During the summer, City Ice is the cheapest option. I think it's $5 for the weekday public session. It's been empty every time I've been there during the week. It's in Long Island City in Queens -- you can get there on the 7 train, and then you walk a bit past the train and it's in a building that looks like a warehouse. Weekend publics can get crowded sometimes, and they won't allow skating backwards when it's crowded, but you can practice jumps/spins in the middle. When we went to a public on Saturday night last month, it was pretty empty. I think their freestyles are about $18 or something, but I've never gone to those at City Ice because I find the area a lil sketchy to walk to at 5am. I think City Ice does have evening freestyles though -- I find the area ok to walk in during the evening.
World Ice is at the end of the 7 train in Flushing in Queens, and you walk about a mile through Flushing Meadows park to get there once you get off the train. It would take you awhile, but the prices are same as City Ice since they're sister rinks.
Freestyles at Sky Rink are pricey, but it'll probably be closest to you. I take the L train to 8th avenue then walk to 23rd and 10th. I've done this walk at 4:30am, no problem. If you're coming from midtown, the E train to 23rd will take you closest. I think it's $25 to $32 for a freestyle session, but I buy them in bulk so I forget. Summer ice means mixed ice on the freestyle though, so it's sometimes high traffic but everyone skates well together from the (supervised) tots to the skaters doing doubles and triples. There's fewer kids on summer ice since they're usually in skating camp.
So basically, if you're staying in midtown, you can take a taxi to Sky Rink in like 10 minutes, or even get there by subway (E) in 15-20 minutes. You'll be able to get back to midtown for work easier too than City Ice. City Ice is in a different borough, but will probably take you 30 minutes on the subway including the walk. It's significantly cheaper, but if you need to be back in the city for work you'll be riding the subway 7 train into Manhattan during rush hour -- not fun. World Ice and Hackensack are just too damn far to be worth it unless you're purposefully going to that rink to check it out. Whichever rink you decide, print out the Google Map directions and you should be fine.
Ok, hope this helps!