The table is made of 2 1x4s. 8ft, and a small peice of floor underlayment plywood. Any thin plywood would do though.
Tools are a drill for screws and any kind of saw. I have a compound miter that makes this very easy but it could even be done with a hand saw. Especially if your hardware store has a scrap section of odd cut plywood like mine.
Price will vary based on local wood prices but I would expect for you to pay less than $25.00 total for wood. I think the 1x4s were less around 4 and the plywood was around 5 for a small peice. Also I used small (1 1/4 inch) drywall screws for this project.
The Lego base plates are on Amazon currently $6.00 a piece but they go up and down. If you can wait watch them awhile. Ebay also has them for good prices.
Total price under $50.00. I do a lot of wood projects so I had plenty of scrap around for most of this project and it came pretty cheap for me.
Step 1: Cut Lumber
Next I cut the 4 peices to make the square box that holds the four 10 inch square plates. for simplicity I didon't miter them, just but connections that are glued and screwed. I cut them a little long (30.5 inches) so the large Lego base plates would fit easily. This step was a 8ft 1x4 also.
Step 2: Attach Legs and Top.
The top of the table is made of 3/16 inch floor underlayment plywood. My local hardware store calls it Lauren mahogany and its thin and cheap. (Bonus is it has high flexibility, I used it to make curved concrete forms for a curved sidewalk) I cut a peice to fit into the box tightly. The edges are held up by small (1x2 inch) peices of the underlay plywood, and then a 1x4 going across the frame. All peices are glued and screwed.
I was surprised how sturdy it turned out using such light duty wood.
Step 3: Finishing Touches
Then to finish it I placed the 4 Lego base plates onto the table. I went with 3 green and 1 blue to possibly be land and water but it can be anything of course being Legos.
On a side note, the whole table was wrapped and became the biggest gift under the tree this past Christmas. Its a great sight to give a huge package to a 7 year old.