In general home repairs are pretty costly, however there are certain common repairs that cost the most to fix, and here is what they are.
Roof: The roof is what protects the interior of your home from the elements. Leaky roofs can cause water damage, mold, and other issues with your home. They must be replaced every 10-30 years, depending on the type of roof. Severe weather like high winds, tornadoes, or hail can cause damage to a roof as well. Sometimes your insurance will pay to have your roof replaced, but if you are paying out of pocket, depending on how bug your roof is and how complicated it was designed, replacing it can cost you anywhere from $5,000 to $15,000 or more.
Foundation: The foundation is what your home is built on, and shifting foundations can cause the entire house to come apart as they progressively get worse. If left unrepaired, foundations problems will cause cracks all over your walls and exterior brick, shifting ceilings (I have seen some that are literally being pulled apart), doors that are hard to open, shut, or lock, cracking tiles, and in more serious cases damage and warping to your roof and busting plumbing pipes underneath your home causing leaks and all kinds of extra work. In the Dallas metroplex, we have shifting soil and are notorious for bad foundations. You are almost guaranteed to have to do some sort of foundation work to your home at some point in time, even if you keep your foundation maintained the way it should be. Pier and beam foundations are typically cheaper to fix than slab foundations, unless the piers under the home are rotting and need to be replaced. Rotting pier and beam foundations can get pretty costly. Slab foundations are the most common in newer built homes, and typically cost between 180 and 350 a pier, or more. Sounds inexpensive, except that foundation repairs are often 20 or more piers and the cost per pier doesn't always include general labor and additional work (like cutting into a deck). Some foundation bids can be up to $10,000 or more.
Heating/AC: The other costly repair that needs to be made to your home is a new heating/AC unit. With a smaller home you can get away with a smaller AC unit, but the bigger the home the more power you will need to heat and cool it. Some homes are so large they even require 2 or 3 heating/AC units. You can estimate that just replacing the unit would be about $5,000 minimum, depending on the brand, size, and reputation of the unit you purchase.