It is 316, not 500, that requires about two and a half digits to write down. 500 requires nearly 2.7 digits.
Short version of the argument that "half a digit" is closer to 300 than 500: If you program your computer such that it interprets a leading 1, 4, or 7 as a 1; and such that it interprets a leading 2, 5, or 8 as a 2; and such that it interprets 3, 6, or 9 as a 3; then when it comes time to transmit a number to you program via GalCom, you can sell a Trit on the market (and send the high, medium, or low leading digit accordingly). Thus, if your number is around 300, then the amount of information you can sell on GalCom is roughly equal to the amount of information that you're using yourself. [work in progress]
Also, $~$10 \cdot 10 \cdot \sqrt{10} \approx 316,$~$ and $~$\sqrt{10}$~$ is the "geometric half" of 10. [work in progress]