You can set the default selling prices of the items you sell in the Set Item Prices window. Then, if required, you can change the prices at the point of sale, which overrides the default prices for that sale only.
Average Cost is calculated as the total cost of an item (the total amount you have paid to purchase the item or the cost entered when you transferred the item into inventory) divided by the number of units of that item you have on hand.
Last Cost is the most recent purchase price for the item.
You can enter unit prices with up to four decimal places. On your invoices, the unit price is multiplied by the quantity sold and the result is rounded to two decimal places.