I'm assuming each quoted amount is in a separate related record--probably displayed in a portal.
Once you've used a check box or other field to "mark" the values they want you can use either a filtered portal or a find to isolate just the selected quote amounts. The basic logic in each case is the same. With a find, you'd specify two criteria (one to identify the original invoice or quote and one to specify just the records with the "selected" value). With a filtered portal, you include a second pair of fields so that only those records with the same parent record ID AND the selected value will appear in the portal.
For more on Filtered portals see: How to filter records to be displayed in the portal?
A field, with multiple values, such as assigned by a checkbox set, can be used in relationships, where each value is almost treated like a 'repeating' value. So if you check the boxes for 2500 and 1000, for a field in the Estimates table, it can show the two related records from the item table, or be used in calculations. Additionally, a dynamic value list could be created based on the quoted quantities and prices.