This response is delayed by three weeks, purely because the picture was not fully formed back in November. Now it is.
"It looks like the 840 Pro is the successor to the 830. The 840 is less expensive because it uses less expensive technology."
After watching the 840s review for a couple of weeks, the standard 840 looks far more like the direct replacement to the 830. The 840 Pro is in a bit quicker class overall than the 830 ever was. This also makes sense in retail as the 840 is a bit cheaper than the 830 was, but the 840 Pro is significantly more expensive than the 830 was.
Is it cheaper to manufacture than the 830? Yes, TLC is cheaper to make than MLC. You get 33% more states (read: storage) for the same material cost. This is a good thing, as we are rapidly hitting the point where SSD cost is dictated by material cost more than anything else. The controller between the two variations of the drive is the same though.
As long as you aren't starving your system with something ridiculous like only having 8GB of RAM, then I would say that an end user would be very hard pressed to tell the difference between the two drive variations, or between either 840 and the 830. Day to day use means a lot of reading and not too much writing, which all three drives are about the same at. Even in the case of writes, having enough RAM means Windows can take it's time on writes without effecting the user experience. Unless your goal is to run a server hosting 100GB database files to 100 users, or you really like copying large video files from one SSD to another, the 840 Pro is a waste. Any extra money spent on the Pro variant would be better spent elsewhere in your system or environment with more effect on the user experience. A vanilla 840 plus a new keyboard or even a new garbage can for the office is a better spend than an 840 Pro by itself.