I got my Sorceror to lvl 15 before I spent any platinum. Even then, I didn't feel like I had to. My advice is:

--Don't feed your pets unless you have tons of money to spare. Potions are more important.
--If dungeons are "empty" like you say, then something is probably wrong. Make sure you aren't accidentally stuck in someone's party from a previous map. (Check the social menu.) Also, when you start a new map, hang around the entrance for a minute so other people can join.
--Definitely invest some points in strength to raise your health. I personally keep my max mana and health roughly equal. (Sidenote: even with this much health, some enemies on elite maps can still kill me in one hit.)
--Get the daily treasure chest for occasional free potions.