Let's start working this out.
I think I'll go with Market Class II, it's a major centre outsized in comparison to its population with regional reach. That's enough to cover most settlements in that corner of the Mediterranean by sea, though not quite enough to reach Roma, Syrakousai or Kart-Hadast (all of which are Class I markets). Nor will it reach the southern coast of Iberia or Sikelia, leaving those places firmly in the grip of Carthage. So that leaves a base income of 9,600gp/month.
Next stop is garrison. Massalia doesn't have one; there are no troops permanently under arms patrolling the borders, watching the city and so on. The city relies upon it's stone walls and its citizens whom it can call up in times of strife. They don't receive pay, it's a duty that comes with voting rights. Indeed they are expected to provide their own arms and equipment, and see to their own training, with only the occasional gathering of the forces to train together.
Massalia's "garrison" comprises citizens of the following types:
300 light infantry (1800gp/month)
240 heavy infantry (2880gp/month)
60 medium cavalry (2700gp/month)
Total: 7380gp/month = 6.15gp/family
That's the nominal value of the citizens who can fight, which would make up a pretty significant garrison if they were all under arms. However, they rarely are, and Massalia if seriously threatened is more likely to see aid from the Romans. Furthermore, the cost of maintaining the collective aspects are voted as a liturgy, so one citizen will pay to gather and train the citizens. So let's say the real value of that garrison is equal to a quarter; 7380/4=1845.
9600-1845=7755gp/month.
Upkeep is 1200gp/month (EDIT: Updated maths as per Alex's errata note). No taxes, because it's independent. No tithes, because there is no state-sanctioned religion and the wealthy are expected to maintain the two temples as liturgies, which might be voted to an individual to perform. Festivals are again liturgies which fall upon one member of the council voted the duty. I don't think I can really justify taking taxes from other settlements, given they don't really have any military power to compel them to. They profit from the trade that reaches Massalia as a result of using those colony-settlements as stop-off points.
That leaves us with 6555gp/month. As above, this is not divided equally between the 600 property-holders. Let's say half (3278gp/month) goes to the 60 wealthiest, netting them on average 55gp/month (the split within that class is likely to be skewed so the richest quartile receive a disproportionate share). A further third (2185gp/month) goes to the 240 next wealthiest, who net around 9gp/month. The last portion (1092gp/month) goes to the remaining 300, who net around 3gp/month.
I like the rationale used under method 6 for generating trade income, and let's assume there are monopolies (because all trade in this era involved them; one of the rights of Hellenistic monarchs was in granting them), but they only add 5% in what is a pretty competitive arena. That gives us 90 x 24 x 300 x .15=97200gp/month from trade. This is more evenly distributed than the revenues, though still dominated by the richer classes. This time the split is 40%/33%/27%.
The wealthiest earn 38880gp/month from trade (averaging 648gp/month), the next tier 32400gp/month (averaging 135gp/month), and the last 25920gp/month (averaging 86gp/month).
That means the total income for Massalia is 103755gp/month, with the wealthiest taking 42158gp/month (averaging 703gp/month), the middle taking 34585gp/month (averaging 144gp/month) and the least wealthy taking 27012gp/month (averaging 90gp/month). That quite usefully gives me a range of what various citizens are worth, by their class. So the wealthy are at the bottom-mid end of Affluent; the hoplite class the bottom of Prosperous and the psiloi class in the middle of Comfortable. Which seems to fit for a rich trading polis.
The PCs employer is in the wealthiest class, he's probably in the 1000+gp/month range, doing better than some of his peers. So his contract with the PCs (forming a liturgy to improve the city's citizen-forces), which I priced at 3000gp for six months work is a significant, but not crippling outlay for him. Especially given the other ramifications (political especially) of being the guy who drilled and trained the militia.
As far as other settlements go, Emporion is class III, everything else nearby is is basically Class V or VI.
That leaves Demand Modifiers and NPC levels to work out. As noted elsewhere, I won't be using the demographic assumption about rules having certain levels; instead there could be any spread amongst the timoukoi from Normal Men up to about 7th level. I'm thinking their employer is 5th or 6th level.