While trying to find a lego table for my daughter for her birthday, i decided all the units were either very poorly made or too small and way to expensive for what you were getting. So i decided to make one.
The one i designed fits over our current table (it is going into our basement as soon as I am done finishing it) and can be made in less than 2-4 hours broken down over a few sessions.
It is a 1/2 inch plywood top (cut to size for free at Home Depot). 2x4s were screwed around the outside edge to ensure the tabletop doesnt slide off the table. I used 4" crown molding around the exterior for a nice finished look.
I purchased 10x10" lego plates and glued them down to the plywood using liquid nails/construction adhesive tubes.
One major point on this project, when you are gluing the lego plates down, you need to secure the plates together using lego blocks - if you dont the spacing will not work out correctly.
My daughter loves this and has not stopped putting her "Lego City" together since she got it.
The most expensive part were the lego plates but you can get them on sale and it cuts the cost pretty good.