That depends on where you live. I'm a professional construction estimator and we have to use a cost index list to calculate what any job would cost depending on the state and city where a job is located. For instance, having remodeling or building done in New York City or San Francisco will cost you 4 times more than the SAME work in Del Rio, Texas. Most other cities fall in the middle between those two extremes but there are still huge differences from area to area, depending on demand and wage rates. Some areas have strong worker unions (thus higher wages) and in some the business climate drives prices up or down.

And unless a contractor inspects your house to see what the construction is and what the layout would have to be for ductwork, there is no way to "guesstimate" what it would cost. It would be a better use of your time to call some HVAC contractors and have them come and inspect the house to give you a quote. They almost never charge for giving an estimate so it will not cost you anything. Be sure and get the estimate in writing with a list of the type of equipment that they will provide. Get at least 2 and preferably 3 separate estimates so you can compare the scopes of work and pricing. If you live in the US, it is worth subscribing to Angie's List to find recommendations from other homeowners for contractors that do good work for reasonable prices.

If its super small, you're better of getting the portable home heaters that kind of look like tower fans or sometimes they have a circular head that looks like a blade fan with no rotating fan inside it of course. They can really make a huge difference in a smaller apartment. My mom used to work in the office in our house and she would close the door and turn on a cheap portable heater and I would walk it and it would be very noticeably warmer than the rest of the house. Plus its a fraction of the cost, and the electrical costs per month to run the heater are much lower than the electrical costs to have a portable heater plugged into the wall or buy batteries for it, if it has that capability. I would think if its a smaller house, which it sounds like based on the 2 bedrooms and a living room, you should be able to have a heat for the bottom floor and the top floor and then the middle would get some residual heat for both, or if you guys spend a lot of time in the middle level you could even get one for the the middle level as well. You should be able to get 3 solid heaters for $200 - $250 for sure. Just the heater and all the ventilation, plus the cost to have it all installed would cost you probably close to $500. And then the monthly cost would be dependent on how much you use the heater. Plus if the heater breaks in the future, you have to pay easily over $100 for parts and probably $50 for labor in order to get the heater fixed, and thats at minimum. Where as a portable heater is easily replaceable because you can just buy a new one. I also suggest you learn to use ceiling fans in the right way, most people don't realize ceiling fans have a warming setting and cooling setting. We all know fans provide a breeze to cool us off, but if you flip the setting to the warming instead of cooling it will rotate the fan blade in the opposite direction (I want to say it makes it rotate counterclockwise but I am not sure). So heat naturally rises because thats physics, but when the fan blade spins the other way, it pushes all the hot air down so it doesn't bunch up at the top. In some places in my house I can feel the line between hot air and and cold air on my face or iic I reach up for something, so I just turn the fan on the warming setting and have it spin at a low rate so I don't get the breeze of a fan, but it still pushes the hair down rather than having that line between hot and cold, you have an average temperature at all heights in the room/house.

If you get a really nice and heavy duty heater, think about getting small and cheap fans or put the heater at one end of the house. Heaters are usually like fans, they only work in one direction, so if you don't have it facing away from the side wall of the house you wight want to get some small/cheap fans to help circulate the hot air around the house instead of it focusing in one room. I think your best bet is it get a really nice and heavy duty heater for the bottom floor and then a single small/cheap fan to help circulate that warmth to the middle level, and some of the heat will gradually make its way up to the top floor because heat rises. And then if the top floor is still too cold, just buy a less expensive and less heavy duty heater to help with the top floor. You should be able to do that for under $200 - $250 easily. I don't know of the best heater for the money in the current market and my mom sold her heater when she no longer worked from home so I don't know exactly which one but I bet there are a ton of reviews out there for you.