Repair and remodeling costs can vary widely depending on your part of the country, local competition among subcontractors, the quality of your finishes, and the existence and number of surprises. Finishes are things like carpet, wall coverings, door quality, and countertops, and can be a significant part of your costs. A paint-grade hollow core door is much cheaper than a solid oak door, for example.
Surprises can increase your repair costs, too. For example, a few years ago I was flipping an older home and had a firm quote of $1,500 to replace all the plumbing fixtures. However, we pulled off crumbling plaster, and revealed pipes that were brittle with age. The plumber also discovered that the entire sanitary sewer line to the street was crumbling and needed to be replaced, the plumbing repair bill went from $1,500 to $6,000.
As you can see, there are really two issues when it comes to estimating: (1) how to estimate your reasonable expenses; and, (2) how to cut down on the number of surprises.