First, Import a saved ME1 game if you can.
Next, when you get the chance, go for hacking extending upgrades if like me sometimes you run out of time before finishing a hack. If not, then forget this.
Do all side missions and look everywhere, because in walk in containers in places like cargo areas on missions, are things like boxes of resources, PDAs that give you money, wall safes etc. Plus Cerberus pays you for doing those Side Missions.
On Horizon, before you do the last section on fixing the satellite tracking system for the gun, go into every house and check everywhere, laptops have money, wall safes, you name it, plus there is ammo to be had.
As the OP said, the DLCs give you lots of money too.