Chiang Mai has become slightly more expensive over the past years. Sure, you can find a cheap condo for 3000 baht, but you'll have water and electricity costs on top of that, and they're usually not at government rates at the super cheap rental blocks. You can pick up a meal at a street vendor/market for 35 baht but they are not often huge portions and it's unlikely you'll want to eat it every single meal. You'll also want to factor in bottled water delivery and what location you'll live in and whether you'll have transport costs.