It's been a few years since I lived up there, but if you eat on the "canteen" and street food end of the food scale, each dish is between 30 - 100 Baht. One plate, pretty standard fried rice or rice and some kind of meat or veg. The next step up is about 120-160 for a meal. And if you eat any kind of foreign food (Korean, Japanese, western, etc) it's expensive on that scale.
Cost of a room is usually from about 4,500/month for a very basic room, to 6,000-10,000/month for a nicer and bigger option. Both of those are fully furnished, one room generally with a bathroom and a lot of places in CM don't have a kitchen but do have a balcony where you could set up a hot plate. Those monthly rates never include water and electric. Water is "per unit" and is maybe 7 Baht a month. Electric depends on whether or not you run your air con a lot. Some rooms will have a water/electric included if it's just for one month but that price is generally more than what you'd actually use. Many places have a 500 Baht fee for the internet, but maybe by now they just add that into the monthly price.
A motorbike is 100-200 Baht per day or 2,000- ? per month. Some places make you leave your passport as collateral, which I don't advise. Bring a copy that you can sign and give them.
Training depends on the gym, but in Chiang Mai they tend to be around 8-10,000 per month. Most don't offer meals with that.
And then you just have to plan for incidentals, like entertainment, tickets if cops target you, etc. I can't tell you how many guys I've seen burn through their 3 month budget in 1 month because they end up eating western food, drinking, and going to the movies when they're bored and didn't plan for any of that. So, just be realistic and plan for it, then stick to your budget.