Online calculator with google spreadsheet

I have a spreadsheet gold price daily (using google docs). i wish someone can help to build form where my visitor can submit gram of item and the form will automatically calculate the total value... The sheet have 10 type of gold, every type have different price, visitor can put gram of item on the form and select type of gold to calculate the price after visitor click submit, the form will take "price/gram" from selected type and give exact value calculation: type 1 "weight item" x "price/gram" ="value1" type 2 "weight item" x "price/gram" ="value2" type 3 "weight item" x "price/gram" ="value3" Total Value="value1+value2+value3" please view my excel