Are you looking for a topper (like something to cover the truck bed)? A buddy of mine from work is actually looking to sell his. We looked quickly on Kijiji and they range from $100 to over $1000 depending on condition etc. Also depends on how big your truck bed is.
As for what you're looking for, there's different types, such as flat ones, or ones that are shaped to give more room.
my FIL had one on his ranger - he eventually got rid of it because it really reduces your visability. i'd recommend that if you want one that you try and strike a deal with the seller to try it on the back of your truck and see if you like it.