I just make them up, of course.
Seriously though, it's the amount of Culture you would accumulated over those turns: +2 per turn while building (from your Palace), +3 per turn after. So turns 2-9 you'd gain +16, and turns 10-11 you'd gain +6, totally 22 out of the 100 you need to expand (22%). That's at Normal speed. At Marathon: turns 2-28 = +54, and turns 29-30 = +6 for a total of 60 out of the 300 you need (20%). Which is a 2% difference (my culture total was off by +2 the first time).
The % of total turns used is nearly identical, too.
The reason the difference is so small is because of the questionable strategy involved.
As someone else already pointed out, you're building a monument in your capital, which is already getting culture from the palace. Not only is this a bad idea in-game, the palace's +2 culture also makes the difference appear to be less.
The bigger reason, however, is that for some reason, you are calculating for the second border pop (100 culture) instead of the first (10 culture), and the first border pop is the only reason anyone would ever build monuments. (Again I'm just echoing itg here.)
Let's look at a much more realistic situation: a newly settled city with no culture (it's not the capital, so no palace). Calculating for first border pop (10 culture on Normal, 30 on Marathon).
On Normal, saving 1 turn on a Monument gives you +1 culture (that is, 1 more than you would if you had not had a Fast Worker). That's 10% of culture needed for border pop.
On Marathon, saving 1 turn on a Monument still gives you +1 culture. But it's not 1/10, it's 1/30...only 3.33%.
To put it simply and more generally: A Fast Worker on Normal saves you 3 times as much time (in game speed) than it does on Marathon. This has been proven. What it does with that saved time is up to the situation, but having 3 times as many saved turns is surely significant.
Marathon actually illustrates the relative insignificance of the FW.
Because FW is really quite insignificant on Marathon. On Normal, however, not so much. And on Quick, it's generally accepted as the strongest UU.