You can estimate wage as (cost of equipment) + (cost of training) / UNIT DIVISOR.
Unit Divisor is 15 for Light Infantry and Slingers, 12.5 for Cataphracts, 11 for Heavy Infantry and Heavy Cavalry, 10 for Bowman, Light Cavalry, and Medium Cavalry, 9 for Crossbowman and Horse Archers, and 8 for Longbowman.
Remember that infantry are assumed to have 45gp in standard kit and cavalry have 80gp in standard kit above the listed weapons. (Exact equipment list is in D@W.)
EXAMPLE:
Horse Archer has Composite Bow (40gp), Short sword (7gp), Shield (10gp), Leather Armor (20gp), Light Warhorse (150gp), Cavalry Kit (80gp), Ammo (1gp), and 85gp in training (again, see D@W), for a total of 393gp. 393 / 9 = 43.6 ~ 45.
If you then decided to equip the Horse Archers with Lamellar Armor (50gp instead of 20gp) and Leather Barding (40gp), cost would go up to 463gp, which would raise the wages to 51.4, ~ somewhere between 50 and 55gp.
Or, another way to think about it, for most units, every 10gp in equipment changes monthly wage by about 1gp. Heavy infantry in chain instead of banded? 11gp/month. Plate instead of banded? 13gp/month. Note that in most cases it's not really worth worrying about.
This isn't actually how I *calculated* the wages for these units (that's a spreadsheet with a lot of factors, and even then it has been manually adjusted in places based on actual play experiences) but it's worthwhile as a rule-of-thumb.