The cold but realistic response to your question is why should a 640 X 480 pixel thermal camera be made available to you for less than $1000. 320 X 240 is still the standard for most applications. There are people with very deep pockets willing to pay significant sums of money for higher resolution cameras. Until that situation changes, there is little incentive to supply 640 X 480 resolution cameras at anything but premium prices.
Also be aware that 640 X 480 resolution cameras do not always have 640 X 480 true pixels. As you may be aware, TESTO used a feature called Super Resolution to create 640 X 480 pixel images from a 320 X 240 pixel resolution detector array. They used the unavoidable movement when hand holding the camera to create a stacked image and tidied it all up in the image processing stage. When mounted on a tripod, Super Resolution does not work.
I have to ask the question.... Why do you need 640 X 480 pixel resolution ?
There are applications for such in areas like the military, science and broadcast programs but 320 X 240 is sufficient for most applications. This is not visible light imaging. Thermal, sources are by nature, less well defined due to a combination of conducted, radiated and convection heat transfer within the DUT. Sharp Delta T transitions usually only occur when a thermal insulator is in play or a DUT is contrasting against a background of differing temperature.
In the early days of thermal imaging the resolution could be quite low and this could make life difficult for the user as they found it hard to spot references against which to interpret the thermal image of an area. On some cameras Visible light camera image merging was used to provide a reasonable context for the thermal image detail. As camera resolution improved, it was possible to see enough detail on the surface of the DUT to establish the location of heat sources and Delta T across surfaces. 320 X 240 pixels are adequate in most circumstances for such image interpretation.
Visible light Image merging was of some benefit but the visible light image could cause confusion with regard to the thermal information in the combined image. FLIR decided to use edge detection techniques to capture edge detail only from the visible light image and overlay only that detail onto the thermal image. The edge detail provides the user with very useful context within an image without producing contrasts that could be confused with thermal data. The MSX system works well in good light but can add noise to the images in low light conditions as the visible light camera AGC increases the gain and produces noise.
Higher thermal resolutions are used in military weapons aiming systems that needed excellent target acquisition and aiming capability at long ranges without a very small FOV. The minimum detectable target size at long range is an issue for defensive weapons systems. Some very high resolution cooled thermal camera systems are deployed on ships etc, but they cost a small fortune and are tightly controlled technology.
Do remember that 640 X 480 pixels just allows you to be twice the distance from the DUT using the same FOV and obtain a similar level of detail in the image. Conversely, if you can obtain the required image at half the original distance you get similar thermal detail of the DUT as when using a 640 X 480 camera with the same FOV at the original distance. I hope that makes sense.
I own X2 and X3 Auxilliary telescope lenses for some of my thermal cameras. These may be fitted with Close Up lenses of 6" and 12" that I also own. These allow me to effectively use my 320 X 240 pixels very close to the DUT and at X2 or X3 magnification. The FOV is effectively reduced by a half or two thirds providing greater detail of a smaller area of the DUT.
If you tell me why you need a resolution higher than the industry nominal of 320 X 240 pixels, I may be able to suggest options that are open to you.
If it just that you want more bang for your buck........ Well I cannot help you there, but can say that you have never had it so good on the thermal imaging front and maybe should just be grateful for what is already available for less than $1000
Fraser