First, a caution.. Allo the devices I mention will look the same from one to the other in that one PVS-14 will look exactly like the next PVS-14, a PVS-7 will look like all the other PVS-7s, and so on. That does not mean that they are all the same. It is the tube in the device that determines the performance, and there are many different tube models that fit in the PVS-7, or the PVS-14, or the Mod 3, or the Micro. It is more about the tube inside the device than the device itself. A really excellent PVS-7 can outperform a PVS-14 with a low end tube. Don't buy anything unless you can determine what is inside it.
The most cost effective way to get into night vision would be to buy a PVS-7 military style goggle. With patience, you can find a good PVS-7 on the used market for $1500 but most sell for $1800 to $2000.
The main issue with the PVS-7 is that the tubes for the PVS-7 tend to have lower performance than the better monoculars. Most notably, the noise level (scintillation you see in the view) will be higher, and the EBI (a background glow) might be higher than you would get in something like a used PVS-14.
There are two major advantages of the PVS-7 over the PVS-14 and those are tha (A), you get to use both eyes (though eye relief is not as good with the PVS-7) and the PVS-7 can be easily converted to work at prime focus or in 2" eyepiece holders, and unlike binoviewers, a PVS-7 will reach focus in almost any telescope with no added magnifcation.
A lot of people that start with PVS-7s and upgrade to a monocular keep their PVS-7s, and a lot of people that start with a Monocular later add a PVS-7, so while the performance is not going to be as good as with a monocular, they can still provide you with the ability to see things like Barnard's Loop, or other nebula you would never be able to see using conventional eyepieces.
The PVS-14s can be found used at good prices, but now you are limited to using it afocally (holding or mounting the device on to a telescope eyepiece.) There are some advantages to afocal, but you can do afocal with other devices besides the PVS-14.
Bob mentions the NV Devices Micro, and you can indeed buy a nice Micro brand new for $3000, but that seemed to be above your budget line and the the performance will not be that much better than a good PVS-7 which costs up to $1500 less. I do recommend the Micro though. It is a wonderful little monocular and is C mount, so you can use it at prime focus, or afocally.
You can shop ebay, or you can also contact someone like Ed Wilcox. Wilcox is not just a seller, he is also a top tier tech, so he knows his tubes. Sometimes he has a really great deal on used or new tubes.