To set the cost for an item by the supplier, edit an item and add the suppliers and their costs in the 'Costs' tab.

This also applies to volume discounts. Click on the '+' button to view and edit these.

'Main' supplier is the default used when an item is added to a quotation or job/sales order, etc..

On a quotation, job/sales order, etc., use the supplier column to change the supplier, then click on the item's menu or the overall 'Apply' button and 'Refresh Prices' to bring through the cost set in the above table.