The default most people go with when equipping troops in basic gear 100g for each troop. So for 500 population, 50k gold, ect.
That would be ok for villages, it's far too low for castles/cities.
For example, this would be medium gear for 1000 troops
Nordic Helmet - 800
Byrnie - 650
Tribal Warrior Outfit - 200
Sarranid Leather Armor - 150
Leather Gloves - 1000
Leather Boots - 850
Mail Chausses -150
Scimitar - 40
Military Sickle - 50
Soldier's Cleaver - 110
Knobbed Mace - 50
Hide Covered Round Shield - 390
Heater Shield - 100
Kite Shield - 210
Dadao - 200
Morningstar - 40
Heavy Lance - 100
Long Spear - 80
Long Voulge - 70
Horn Bow -120
Rus Bow - 80
Arrows - 600
Arbalest - 20
Bolts - 80
Rouncey - 150
Medium Wooden Ladder - 100
Siege Shields - 50
Construction Site - 5
Construction Material - 150
Cost: 160,708And since castles and towns shouldn't be that easy to take I'd go with 200k/1000 troops so they look like a decent army and 75k/1000 troops for villages to make it more like a militia.
As for population numbers, I'd go with 500 for villages, 1500 for castles, 2500 for towns.