Several factors affect the value of a 99/4. Early in the production run there were a couple of variants that did not last long. All of these used a different power supply and most of them had a slide switch instead of the Solid State Software label at the front of the cartridge port. The slide switch was for the internal speaker under the grille behind the cartridge port. This was dropped pretty fast. All total, TI sold something in the vicinity of 50-75 thousand 99/4s, according to a news article I read back in 1981 on computer market share. The article concluded that there was no good reason to buy one because there would never be much of a third-party software market for a machine with such low sales numbers.
Condition and presence of the original box also make a difference--very few 99/4 boxes survived, so they've become somewhat of a collector item.
As Curtis noted, $75-$100 is a good range for the later versions, with the earlier ones with the cartridge port slide switch going for closer to $200. I have both major varieties in my collection.
One other item of note: machines with manufacture date codes in 1979 are usually higher priced in their category than those with later codes, and both varieties were in production together until at March 1980 at a minimum, as my slide switch keyboard was made at the beginning of that month.