I don't know about Red Spirit stones being not *that* expensive. They kinda are? A single stone is 100 silver which can feed a mortal family for some time. A month, or a full year. Maybe more, depending on what quote you pick/where Yrsillar changed his mind about the economy. (Note, it's perfectly fine for him to keep the economy somewhat woobly or change his mind at times; this is a fantasy quest, not a economics simulator.)
But the fact remains that a single Red soldier requires several Red Stones to train, meaning it is a huge investment going by these numbers. I doubt a town of 2k could support 150 Reds. Maybe 20, a bit more if they were at full war footing. From my understanding though, the town is both the local strong point, housing the main military force and also at the frontlines thus defended more strongly than usual. What this means is that I wouldn't look at the 2k town population to square them up with the garrison, but include the peasants and outer villages which easily could be ten times the people or more and a two digit number of Cultivators.
Furthermore, since the town is close to the border it probably does have further military support from the interior of the province. So I can totally see the number of reds, under the assumption that it totally isn't the "average of the population" or anything like that.
What is much more surprising to me is the Green-5 captain and the 'handful' of Green officers. I didn't think that you'd necessary have more than one Green, maaaaybe two, in a hundred Reds. Judging from the narrative so far, and how we learned very early in the previous quest that Late Red was average for inner city guard officers.
Maybe I am just wrong, or it is simply the same effect as above. The high amount of Greens might simply be drawn from the interior of the province and it really is more of a 1 Green to 100 Reds ratio. Or not. I doubt Yrsillar really wants to go into too much detail there and give hard numbers.