While it is true that TTN is definitely not a public transit-friendly airport, don't believe all of what Jerseyguy said. I flew into/out of TTN 20 times last year, and with the exception of two or three times when it was extremely cold and/or pouring down rain or snow, I never paid more than $1.50 to get between Trenton Transit Center and TTN airport (the cost of the local NJT bus fare).

You have two options once you get to Trenton Transit Center:

Option 1:
Take the 608 bus directly outside of the train station. Once you get outside, walk straight across both streets and turn right. The 608 bus stop is right next to the gated parking lot there.

Get off at Grand Ave and Upper Ferry Rd, where the Dunkin Donuts/7 Eleven/gas station are. From here, head north (the same direction the bus was heading; the bus will make a turn when you get off, but walk in the same direction that the bus had been going before the turn)

You will be at the edge of the airport grounds within 10 minutes. However, the walk to the terminal is another 10-15 minutes. Unfortunately, there is no sidewalk here but it is not a terrible walk across the vast grassy field, and you only need to reach the parking lot before you are walking on paved ground again. Total walking is ~0.9 miles from where you get off the bus to the terminal entrance.

Option 2:
This is the option that Jerseyguy mentioned, but there ARE sidewalks from Trenton Transit Center to the 607 bus stop. I walked this way numerous times and while Trenton is not an area you'd want to live in, from the train station to the bus stop is not that bad. You're walking in the direction of the Marriott, the most expensive hotel in the area, where a lot of people stay when they are visiting the nearby state government offices on business.

When you walk out of the train station, head left and continue straight at the intersection where the River Line light rail is. You should be walking parallel to the parking lot, on Barlow St. Keep walking straight (Barlow St will basically tun in to Market St) until you hit S Broad St. At S Broad St and Market St is the 607 bus stop. This is like a 10-minute (0.5 mile) walk.

The 607 will take you all the way to Bear Tavern Rd and Cardinal Dr. You will see the airport when you get off. However, you still need to make the extra 0.5 mile walk to the terminal building.

So you can either get on a bus right away, and then walk a mile to get to the airport. Or you can walk half a mile to the bus, and then walk another half mile to the airport. I prefer the former, since the area around the airport is pretty nice.

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Now, the bus itself (either the 607 or 608) goes through some pretty low-end areas, and the passengers won't exactly be on par with the professional white collars you might be used to on the Manhattan M42 route, etc. However, in my experience these are very lightly traveled routes so you will easily be able to just grab a seat and be comfortable until your stop. And once you pass through the bad areas, the area where you get off is incredibly nice. The airport itself is in Trenton suburb Ewing Township, one of the most upscale suburbs in all of New Jersey.

All that being said...... if the difference between LGA/JFK/EWR and TTN is only $20, and you don't mind dealing with the madness of those other airports, just pay the extra. I loved TTN because fares to/from CMH (now a discontinued route) were almost always $39 or $49, or even less with a discount code (vs. $150+ for the NYC airports). I also very much appreciated not dealing with the hassle of LGA/JFK/EWR, which I really burned out on after living in the city for five years. The walking is not that bad. It's almost serene since you'll likely be the only pedestrian as far as the eye can see. So just pop in the headphones, listen to music, and enjoy the stroll to the airport where you are at a ticket counter/machine within 5 seconds, virtually every employee at that time will be focused almost exclusively on your flight, and your wait on the tarmac for other flights ahead of you will be exactly 0 minutes.

Hope that helps!