Anyone has tips to level up my gear? sometimes even leveling up 3 levels is expensive as fuck, and I can't imagine leveling up all my gear to level 70
The cost of upgrading gear doesn't change depending on the number of levels you're upgrading, so as a general guideline, only upgrade when you need it.
Legendary armours are all part of a set, and obtaining the last item from a set upgrades the entire set to the level of the last item you obtained. You may therefore want to avoid spending any money upgrading any legendary armour from an incomplete set, as the game will do it for you later for free.
Each mercenary tier has an associated perk. At tier 3, upgrade prices are reduced by 50%. Other tiers reduce the cost of buying items at blacksmiths and upgrading the ship, so all of these are money-savers that will make upgrades easier.
By later in the game, money should start piling up, so the biggest roadblock to upgrades will not be money, it'll be resources (for me it's specifically leather, and I don't imagine it'd be much different for most others). All blacksmiths sell resources so once money stops being a problem it's a good idea to stop at every blacksmith you pass and buy any resources they have.
You probably already know this but on the quest boards in each town and on the Quests tab in your menu, each quest has three icons showing the approximate level of XP, money and resources you can expect as a reward for completing the quest, with the more valuable reward levels coloured gold. If you're really stuck for resources then focusing on quests with good rewards might be an idea, though I think it's usually best to go for XP.
Many locations have war supply crates, the location objective will be to burn them but they can all be looted first. They don't contain massive amounts of resources but every little helps.
