Select the Preferences. In the Custom Features section you will find a section titled Estimate/Update.
In the field titled Estimate Unit Sales Price, click on the arrow by this field to open a list of custom fields. You should see a choice named EstimatePrice – click on this to select it. If you wish to use a different custom field to hold this information you can select that. If you wish to create a different name for this field you can type it in directly.

Note that there is a limit of five custom fields that you can create in QuickBooks here (this is being added to the item list). If you already have five custom fields defined you won’t see the EstimatePrice name, and you will have to choose an existing field (or not use this feature).
Entering a value here enables the custom feature. To disable it, erase the name from this preference.