The expression ballpark figure is an American slang term that means an estimate or guesstimate. If a cost or amount is considered in the ballpark, then it is within the range that the person asking for the estimate expected. If not, then the figure is of course out of the ballpark. A ballpark figure can be asked for as a price estimate or in many other situations.
One example of needing a ballpark figure is in the early stages of planning an event such as a wedding. Information and the estimated cost of each aspect of the ceremony, reception and honeymoon must all be gathered before decisions are made and actual amounts worked out. In order to stay on the budget set for the wedding, the couple will need general figures for everything so that they'll know what will fit in their budget and what won't. In most cases they’ll also need a figure to use for their number of guests if they haven’t received all the invitations back yet.
A ballpark figure is handy whenever the exact amount of something isn't yet known. For example, if a homeowner asks a plumber for an estimate, the plumber looks at the job that needs to be done and gives the owner a general figure of how much the work will cost. The actual amount on the invoice will be calculated to the penny in terms of labor, parts and taxes so it will differ from the original figure. Many such figures don't include tax.
Before agreeing to any type of work based on a ballpark figure, it is a good idea to have signed paperwork stating that your bill is not to exceed a certain amount without your further approval. You can then get actual itemized costs to be sure you're being charged a reasonable amount. Getting other estimates before choosing a company to do any type of work is also a good idea. This way you can get a much better idea of what a reasonable figure would be for what you need done.