overestimate

An overestimate is when someone believes or says that something will cost more, take longer, or require more effort than it actually does. It's like thinking your new phone bill is going to be way higher than it really is.

When we use the word "overestimate", we usually mean that someone has made a mistake about how big of an issue something is. For example, if you think a project at work will take three months to finish but it only takes one month, you've overestimated the time needed. This can happen with money too - maybe you expect to spend way more on a new house than you actually do. When someone says "I overestimated my budget for this trip", they mean they thought it would cost much more than it really did.