This might help:
http://www.wizards.com/default.asp?x=dnd/rg/20050816a
Posting relevant parts here for BOZ who is blocked from WOTC's site:
Building a Construct
The easiest way to make a construct requires a suitable object and an animate objectsspell. If you're not satisfied with a mere animated object, however, you have to put in a bigger effort.
Creating a fairly elaborate construct such as a golem or shield guardian requires the Craft Construct feat (described on page 303 in the Monster Manual). The process of construct creation is just like creating a magic item. The process is described in Chapter 7 of the Dungeon Master's Guide and in Rules of the Game: Making Magic Items. Here are the highlights:
Prerequisites: A construct has a list of prerequisites, which is included in the creation section of the construct's description. A list of prerequisites might include one or more feats, spells, and miscellaneous requirements such as level, alignment, skills, and race or kind.
A construct's creator must have a caster level high enough to cast any prerequisite spell the construct has.
In most cases, the construct's creator must provide any required spells personally; you can't have another character cast them for you, but you can use a scroll that you activate yourself.
Cost: A construct's description (usually) includes a market price and a cost to create the construct. To calculate the creation cost for a construct, subtract the cost of any special materials the construct requires from the market price. Divide the remainder in half. The result you get represents the basic materials you must buy to build the construct. This basic cost includes the cost of the construct's body. Most construct descriptions include a separate cost for the body to allow DMs and players to use the Craft skill to create the body.
The total cost to create the construct is the basic cost plus the cost of special materials. For example, a construct with a market price of 100,000 gp and 10,000 gp worth of required special materials has a creation cost of 55,000 gp. (Here's the math: 100,000 - 10,000 = 90,000; then 90,000/2=45,000; then 45,000 + 10,000 = 55,000 gp.)
Making the construct also requires experience points equal to 1/25th the market price minus the cost of special materials. The construct from the previous example has an XP cost of 3,600.
You can make an advanced version of a construct (one that has more Hit Dice than shown in the creature description). Each extra Hit Die adds 5,000 gp to the construct's market price. If you add enough Hit Dice to increase the construct's size, add an extra 50,000 gp to the construct's market price.
Time: For every 1,000 gp in a construct's market price (or fraction of 1,000 gp), the creator must spend one day working on the construct. The construct from the previous example would require 100 days of work.
Environment: Creating a construct requires peace, quiet, and comfort, just as preparing spells does (even when the creator doesn't need to prepare spells). Any location a character uses for construct creation also must have enough space to hold any special equipment and materials the construct requires.
Equipment: Some constructs also require a specially equipped laboratory similar to an alchemist's lab. The cost for setting up such a laboratory (if it is required at all) is given in the construct's description. The cost for a lab is not included in the construct's market price or base price. Once you set up a lab, you can use it over and over again.