My favorite aviation answer; it depends. How much things cost depend on lots of factors. When I owned an airplane, it was in the midwest so most of the costs are on the lower end compared to the northeast or west. Where I lived, ramp fees were 50.00 a month, an open hangar was 100, an enclosed hangar ranged from 220-330 for a single, the higher side of those hangers could also fit a twin such as a 310. Some other costs, a vfr transponder inspection was 100.00 and a IFR transponder/ pitot static certification was 175.00. An annual is a big question mark, it depends alot on the mechanic you choose and the condition of the aircraft. I had annuals on a light single from 400.00-6000.00. The more expensive one had 2 cyclinders replaced at about 1100 a piece. Even counting that, you can see that the price varies alot. In my case originally I just went to whatever mechanic seemed like a good one. Later I discovered an older mechanic who was doing it more for a hobby, I'd work with him to keep him company and traded him some flight instruction time. That's how I kept the cost down later. I also trusted his work more than any other mechanic and knew my aircraft much better.
Aircraft parts are downright expensive, take what you'd pay for a similar car part and multiply it by somewhere between 5-10. For example, an alternator for your car you'll pay something like 60-100 bucks, an alternator for an airplane you're going to pay somewhere between 300-600 depending on the alternator.
As for insurance, I paid something like 700-900 a year for a 30k hull. Here again that depends on lots of variables, this was a fixed gear tricycle airplane. You'll pay more for a retract single and my buddy who had a 310 paid more like 6000 on a 50k hull, so it goes up fairly significantly to a twin.
Somebody else already said it, but he needs to identify his mission. I'm not a huge believer in flying a multi just for the heck of it. Unless he has a reason for that you're just unneccesarily increasing all the costs, bigger hanger, higher insurance, more expensive annuals, and two engines to care for and feed.
If you have any other questions, I can try to fill you in on some more of the costs. I can also suggest that he join the owners group for whatever plane he's attempting to buy before he buys one, so that he can figure out what sort of issues he's going to be looking for, as well as the subtle difference between model years. Some research now can save him alot of headache later on AD's and general issues that should have been complied with already.