I have a one year old Shiba Inu. I would say in a month I spend anywhere from $60-100 on things for him. That is, of course, not including the hundreds of dollars of damage he's done to various items or vet bills (puppy stages are the worst).
Having him is a lot of work. Ludo is really high energy and is prone to getting really bored. And a bored Ludo makes everyone miserable. So, my boyfriend and I run with him for 45 minutes to an hour each morning, then will often take him to the dog park in the evening or on another run. On the weekends, we're usually hiking or doing something outdoors with him. We try not to leave him alone in the apartment for more than 3-4 hours at a time, so we have to build our schedules around that. Luckily it's fairly easy for me to come home during the day since I'm a grad student, but working a 9-5 would make it difficult.
Ludo also can't be trusted off-leash under any circumstances ever. If he does get off leash (which he's done before by darting out the door or slipping out of his harness), then we get to spend at least 45 minutes running across traffic, through the neighborhood yelling his name begging him to stop. We never catch him until he wants to be caught. I also took him to puppy classes and enrolled him in adult obedience classes a few weeks ago. Sometimes, if he feels like it, he'll even do the tricks that he learned instead of glaring at me for giving him commands.
The one easy thing about him has been house training him. He had one accident the first day I brought him home and he has never used the bathroom in my apartment since. That being said, I'm super lucky in that. Most people who get puppies go through months of house breaking.
There's a huge difference in getting a rescue vs getting a puppy from a breeder. With a puppy, you don't really fully know it's personality or the problems you'll encounter with it. With a rescue dog (assuming it's not a puppy), it's personality is usually more well known and any behavioral issues are being worked through (if not solved). I would always say first time pet owners would be smart to get a rescue dog. That being said, I didn't follow my own advice and it's worked out well.
My dog is our only pet and he doesn't seem to get lonely. But he loves trips to the dog park so he can play with other dogs! That helps a lot, I think.