As with all grids in Workbooks, you can customise the layout to show the columns that are appropriate to the Opportunity, including columns relating to cost and any custom fields you may have created for Line Items. Information on customising grids can be found here.
You must enter at least one Line Item but can add as many more as needed and you can modify the values and quantities as you learn more about the Opportunity.
Line Items can reference products/services in your Product Book, which will pull through any price/cost/tax information already entered there, although this can be overwritten, if required. Alternatively, you can simply free type in a Line Description and all values.
Each time you add a Line Item, the total figures for Net Amount, Gross Amount, Gross Margin and Total Cost are updated. Similarly, each time you Save the Opportunity the figures behind the Analysis Reveal are re-calculated.