Cost Co which is like Sams Wholesale, has a pack of Six with edges for $10.99
http://www.costco.com/
And if you use the edges and glue them vertical and cover with VINYL J MOLD(like drywall corner mold from Home Depot or Lowes), you can make a huge 18x8 foot, large oval track for around $70.
VINYL J MOLD $1.19 for like 12 feet
It slips right over the extra foam edges you get with the tile packs
http://www.tamlyn.com/pvc/jmold.html
The reason I say cover the side foam rails with plastic, is that foam grips the car on impact and stops you. I prefer bouncing of the rail and staying in the race.... at that price of scratching up the car body. Which is why I have a race body and a show body.
And another tip is that a hot glue gun holds the foam tiles and edges together like steel. Not to mention how easy the foam tiles are to cut any shape you need(ex. round corners).
I hope to to finish my track this weekend, and post some pics.