support converting amounts to cost, using their saved price.

And do this by default for auto-balancing transactions, as ledger seems to.

Ledger/Amount.hs

@@ -71,6 +71,14 @@ amountop :: (Double -> Double -> Double) -> Amount -> Amount -> Amount
 amountop op a@(Amount ac aq ap) b@(Amount bc bq bp) = 
     Amount bc ((quantity $ convertAmountTo bc a) `op` bq) Nothing
 
+-- | Convert an amount to the commodity of its saved price, if any.
+costOfAmount :: Amount -> Amount
+costOfAmount a@(Amount _ _ Nothing) = a
+costOfAmount a@(Amount _ q (Just price))
+    | isZeroMixedAmount price = nullamt
+    | otherwise = Amount pc (pq*q) Nothing
+    where (Amount pc pq _) = head $ amounts price
+
 -- | Convert an amount to the specified commodity using the appropriate
 -- exchange rate (which is currently always 1).
 convertAmountTo :: Commodity -> Amount -> Amount
@@ -137,6 +145,10 @@ normaliseMixedAmount (Mixed as) = Mixed $ map sum $ grouped
       symbols = sort $ nub $ map (symbol . commodity) as
       hassymbol s a = s == (symbol $ commodity a)
 
+-- | Convert a mixed amount's component amounts to the commodity of their
+-- saved price, if any.
+costOfMixedAmount :: MixedAmount -> MixedAmount
+costOfMixedAmount (Mixed as) = Mixed $ map costOfAmount as
 
 -- | The empty simple amount.
 nullamt :: Amount

Ledger/Entry.hs

@@ -76,8 +76,9 @@ isEntryBalanced (Entry {etransactions=ts}) =
 -- | Fill in a missing balance in this entry, if we have enough
 -- information to do that. Excluding virtual transactions, there should be
 -- at most one missing balance. Otherwise, raise an error.
+-- The new balance will be converted to cost basis if possible.
 balanceEntry :: Entry -> Entry
-balanceEntry e@(Entry{etransactions=ts}) = e{etransactions=ts'}
+balanceEntry e@Entry{etransactions=ts} = e{etransactions=ts'}
     where 
       (withamounts, missingamounts) = partition hasAmount $ filter isReal ts
       ts' = case (length missingamounts) of
@@ -86,5 +87,5 @@ balanceEntry e@(Entry{etransactions=ts}) = e{etransactions=ts'}
               otherwise -> error $ "could not balance this entry, too many missing amounts:\n" ++ show e
       otherstotal = sum $ map tamount withamounts
       balance t
-          | isReal t && not (hasAmount t) = t{tamount = -otherstotal}
+          | isReal t && not (hasAmount t) = t{tamount = costOfMixedAmount (-otherstotal)}
           | otherwise = t