Although you can certainly plan for known maintenance costs around the house, including annual services like HVAC cleaning and inspection, flushing the water heater, cleaning the gutters, and so on (although some you can also do on your own to save cash), there's no way to accurately prepare for home repairs, since you don't necessarily know what's coming down the road. Of course, you may have some idea when a major job like roof replacement is on the horizon, and home improvement projects are something you can probably plan for, as well. And as for unknown repairs, even these can be planned for to some extent simply by putting some money into a rainy day fund. But once you start planning for specific repairs or improvements, there's no reason you can't determine a rough estimate for the project based on a variety of factors and sources of information.
The best way to figure out how much a home improvement project is going to cost is to get bids from several contractors. With referrals from friends and sites like Angie's List, just for example, you can meet with contractors to describe the project you're considering, whether you're repairing a portion of your home, upgrading materials, or completely gutting and renovating rooms. And you'll likely get a range of bids that will help you to figure out a median cost. Of course, there are other things to consider besides the number. For example, you want a contractor that is licensed and bonded, which may entail a slightly higher price tag. And you'll certainly want someone that you feel is reliable and trustworthy. But with several bids to average you should be able to work out a suitable budget estimate for whatever repair work or home improvements you have planned.
But keep in mind that the bid for any project is just the starting point and it is very unlikely that your final cost will match the estimate exactly. What you can probably find out with some accuracy from your contractor is the cost for labor (hourly or daily rates, for example), as well as the projected time for the job. And when you hire a crew you should be able to sign a contract that includes a set price for labor. From there you have a lot of wiggle room when it comes to the cost of materials. Here, too, your contractor should be able to give you some idea of potential prices for various materials based on the square footage. For example, he can probably give you a basic range of prices for granite, hardwood flooring, and so on.
However, prices for materials can vary widely, and you have so many options to choose from that you might want to do some research on your own to get a better idea of cost. In terms of flooring alone, you could be looking at vastly different prices between hardwood, reclaimed hardwood, and laminate, and that's not even including carpeting, tile, or other options. And when you know the square footage you want to cover you should be able to get a fairly close estimate for materials, so long as you're willing to do a little legwork. You may also have to factor in equipment and machinery rentals like compressors and pneumatic tools, wet saws, or even LA cranes for rent, although again, your contractor should be able to spell out these costs for you. But with a clear idea of the project you want to undertake, with bids from contractors, and with prices for materials, you can probably come up with a good estimate for your repairs or home improvements.