Can I actually support myself with a greenhouse?

no way to know how much it costs to live comfortably because I don't know where you live. Cost of living can vary greatly from city to city even within the same state.
Also, whether you could live on what you make depends on if you figure out what to sell during the winter months. there are green houses in my city (Rochester MN, zone 4) that are open year around but they also have a floral department and sell house plants, etc.
doesn't matter how much it costs to own a horse, how would you make money owning one??
I would suggest you go to college for botany and horticulture. those people work in all sorts of things to do with plants. Land reclamation(NDSU Experiment station works with native grasses to reclaim coal mining land ), colleges that work developing new fruit and veggie varieties( Honey Crisp apple developed right here in Minnesota!) and all sorts of things. Knowledge is power.

It seems to me that you are going to be a junior in the high school while doing part-time job in the greenhouse.
About your three questions; namely, how much do it cost for a comfortable life. It depends on what living standard are you expected. Secondly,how much would a small greenhouse business bring in. A small greenhouse business doesn't make much money nowadays. Thirdly, how much do horses cost to own and keep them healthy. Usually, the purposes of people keeping horses are for riding, racing and gambling, they need to have a lot of investment in order to be enjoyable and winning the gambling and that mightn't be easily supported by ordinary people. In view of my own opinions, it is not advisable to carry on according to your planning unless you might have another better idea to earn a living.
Yip

I doubt it.

You can, it's getting the greenhouse that will cost , not to mention the energy to run it . Wisconsin is not the warmest of states.

A greenhouse could bring in some money especially if you find unusual niche markets, or high value crops.

Ginseng is grown in Wisconsin, this generally needs shades rather than greenhouses.It also has a reputation for completely exhausting the soil it is grown in (although this may be a myth spread by the people getting rich selling it).

I once heard about a farm growing a type of rhubarb used for natural dying.

One high value greenhouse crop which springs to mind carries a six month sentence in Wisconsin, but this might change in future (along with the potential profits) :-)

My point is that if you grow fresh strawberries or tomatoes, you may be able to turn a profit, but you will most likely have big competition. If you grow something unusual (something used in Chinese/herbal medicine perhaps, unusual fruit/nut trees, or young plants which are difficult to grow from seed), you may find a profitable niche.

Horse manure is useful in greenhouse culture for making hot beds, but it isn't as good for feeding plants as other manures, even when well rotted in my experience. Horses can be obtained cheaply in some cases (unsuccessful race horses for instance), but they can be quite high maintenance (unlike donkeys which are also nearly always cheap to buy and feed), they need a lot of good grass, and in the absence of other grazing animals the pasture generally deteriorates. Riding schools can be profitable, but watch for insurance costs, horse riding causes A LOT of accidents.

With one greenhouse... probably not. With several, possibly. You need a way to distribute your plants across a good sized area for enough sales to take place to keep you in the money. I probably wouldn't try to do it in Wisconsin though. You need dense population areas nearby so that you can also grow organic veggies for the markets.

There is a fellow I saw on YouTube that "rents" peoples yards and grows organic greens and makes a fair living off of it. He never mentioned a greenhouse in the video I saw. Renting space does seem to have some advantages. You don't have to invest big money in land to start.

Being that you are still 15 I suggest you take some business classes. Even if you decide against your idea they can still come in handy in the future.

Yes, depending on your choice of plants, the size of the greenhouse and your means of heating it. Grow transplants of tomatoes, other desirable vegetables. Zone 4-5 is tricky you have a short sales period and buyers a short growing season. Look for sources of short maturing varieties. You will probably need a regular job, at least for a while.

This sounds better. A greenhouse is supporting millions of people worldwide by accelerating food production. I want to know that who gave the idea of GREENHOUSE. A greenhouse could mean a house which would not cause harm. We must change the name from a Greenhouse to a Blackhouse so that people would know the fact. This is supporting us but the the future of our future generation is wrapped in gloom and anxiety.We must once think.