Considering the simplest camera is a monochrome one, and also because it doesn't output 3 color channels (only 1 byte per pixel), it seems monochrome would be the best, so lets see what the bit rate would be. Starting at 1920x1080 at 240fps, that gives us a byte rate of 497,664,000 bytes per second. If the resolution is reduced to 640x480 that gives a frame rate of 1620fps. Further reducing the resolution to 320x240 that gives us a frame rate of 6480fps. And even further reducing the resolution to 160x120 gives us a frame rate of 25920fps. That is very impressive. But what is the bit rate? The byte rate is 497,664,000 bytes per second, and a byte is 8 bits. So the bit rate is 3,981,312,000 bits per second, which is 3.98GBits/s. That's a truly high bit rate, but it's nothing that a cheap FPGA and a few gigs of RAM can't handle. All you need is a simple array of RAM chips (usually these will be on a small memory board like those in a PC). An 8GByte RAM board is pretty cheap. It costs less than $100, and would give you about 17 seconds of recording time at the above mentioned byte rate. FPGAs have a very wide range of prices. Some cost thousands of dollars, while I've seen others for under $10.
It should EASILY be possible to manufacture a simple high speed camera, with the above mentioned specs, for between $200 and $300, and then make a decent profit by selling it at a price between $500 and $1000. Yet no company has done so. They all go overboard with specs (producing devices that cost well over $10,000, and some even over $100,000), using application specific (and quite expensive) in-house manufactured components (rather than cheaper off-the-shelf components) to get insane specs like full RGB color video at 640x480 at 4000fps. Yes, there is probably some scientific demand from major universities, the military, and other similar research facilities for such equipment, but it completely ignores another sector of the economy, the average consumer who happens to have a hobby/interest in science. In my opinion, major manufacturers of high speed cameras need to branch out, and in addition to their continued operations that support the professional scientific community, they need to create a separate product line that targets the average consumer. Such a cheaper product line for the average consumer, could be accomplished by making a high speed camera with specs similar to those that that I have mentioned at the top of this thread.
Thermal imaging camera manufacturers like FLIR have already branched out, and made cheaper consumer-level options available. Some companies like Seek have even started up with the sole purpose of creating cheaper thermal imagers that the average consumer can afford. Yet there are no such companies popping up making cheaper high speed cameras, nor are any of the major high speed camera manufacturers creating separate consumer product lines. There's a demand out there for such equipment, but no company seems to be trying to fulfill that demand. In my opinion, they could make a lot of money by targeting consumers with cheaper versions of the products they make.