There is no realistic value. I don't know where you are getting that from.

If there are 5 people in one market and there are only 2 customers who are going to buy 20 pieces/week:

A- 5 item
B - 6 item
C-100 items
D - 10 items
E -15 items

On the market board and we know that the cutoff price for profit is at say 40 gil.

A - x2 - 100 gil
B - x3 - 90 gil
A - x3 - 88 gil
B - x3 - 80
C - x10 - 77 gil
D - x5 - 56 gil
C - x10 -55 gil
D - x5 - 50 gil
C - x10 - 45 gil
E - x10 - 44 gil
C - x10 - 43 gil
E - x5 - 42 gil
C - x10 - 41 gil
C - x10 - 41 gil

Now C doesn't have to worry at all because he doesn't need to relist, hes making money period and now controls the market. Everyone above him is boned because if they had to relist with a 5% fee, they would be losing gil. We've seen this situation 1 too many times.