Help with determining cost of goods sold

[Brizzle Mc' Nizzle]

I'm stuck on a query and, obviously, looking for some help. I am trying to keep track of the cost of the number of television wall mounts I have sold to individuals over a period of time. (The period of time currently being past to present) In doing so, I am trying to develop a query that selects the quantity of mounts sold from a table called `sales` and the cost of the inventory that was sold from a table called `inventory`.

Here are what the two tables look like: (both images leave out the `id` and `model` fields.

`sales`


`inventory`


The problem I am having is that if the number of sales exceed the quantity in the first row of the `inventory` table with a cost of $17.62, I don't know how to continue on to the second row so that I can subtract revenue from the new cost, which is $11.73. (The cost represents the price/mount) Anyone have an idea of how a query like this would work?

tl;dr: I need help developing a query that can determine the cost of the inventory I have sold using a first in, first out approach.

Example: If the quantity in the `sales` table was 15, this query would calculate that 1 of those mounts, the quantity in the first `inventory` row, cost me $17.62 and the remaining mounts would have cost me $11.73 since the first shipment of mounts are now all sold.

-- phpMyAdmin SQL Dump
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-- Host: localhost
-- Generation Time: Aug 03, 2012 at 08:17 PM
-- Server version: 5.1.63
-- PHP Version: 5.2.17

SET SQL_MODE="NO_AUTO_VALUE_ON_ZERO";
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/*!40101 SET @OLD_CHARACTER_SET_CLIENT=@@CHARACTER_SET_CLIENT */;
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--
-- Database: `tay_wallmount`
--

-- --------------------------------------------------------

--
-- Table structure for table `inventory`
--

CREATE TABLE IF NOT EXISTS `inventory` (
  `id` int(32) NOT NULL AUTO_INCREMENT,
  `model` varchar(32) NOT NULL DEFAULT 'APTMM2BC',
  `quantity` int(32) NOT NULL,
  `cost` float NOT NULL,
  `date` timestamp NULL DEFAULT CURRENT_TIMESTAMP,
  `out_date` timestamp NULL DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=MyISAM  DEFAULT CHARSET=utf8 AUTO_INCREMENT=5 ;

--
-- Dumping data for table `inventory`
--

INSERT INTO `inventory` (`id`, `model`, `quantity`, `cost`, `date`, `out_date`) VALUES
(1, 'M2APBCTM', 1, 17.62, '2012-07-28 18:30:13', NULL),
(2, 'M2APBCTM', 36, 11.73, '2012-08-02 05:01:07', NULL);

-- --------------------------------------------------------

--
-- Table structure for table `sales`
--

CREATE TABLE IF NOT EXISTS `sales` (
  `id` int(32) NOT NULL AUTO_INCREMENT,
  `model` varchar(32) DEFAULT 'APTMM2BC',
  `quantity` int(32) DEFAULT NULL,
  `price` int(32) NOT NULL,
  `date` timestamp NULL DEFAULT CURRENT_TIMESTAMP,
  PRIMARY KEY (`id`)
) ENGINE=MyISAM  DEFAULT CHARSET=utf8 AUTO_INCREMENT=41 ;

--
-- Dumping data for table `sales`
--

INSERT INTO `sales` (`id`, `model`, `quantity`, `price`, `date`) VALUES
(1, 'M2APBCTM', 1, 35, '2012-07-30 19:29:57');

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Edited 6 time(s). Last edit at 08/08/2012 07:02PM by Brizzle Mc' Nizzle.