Price options provides the option to lock prices in a quote/order. If you wish to set an order at a specific value you can "lock" any of the totals. When a total is locked, any product editing (adding or removing products) does not alter the price.
Begin in a quotation, in Edit mode.
Click the Price Options button on the bottom toolbar:
Place a tick in the box to Lock Prices.
You can click in any of the boxes to set a new selling price.
When you have made a change you will be prompted to recalculate the totals. Click Yes and the other totals will be immediately recalculated.
Click Update to return to the quote.
Note that if you have a locked price and show individual prices or sub-totals on the quote printout they will not add up to the grand total. You need to change the print preferences so you only show the grand total. The total on screen will show a padlock 
when a price has been locked.
The costings screen will also inform you that the prices have been locked and normal calculations do not apply. Individual item prices will not be shown as they will be irrelevant.
Price Options should not be used if you enable invoicing from the delivery note.