The picture at the top is the type of trap I used to build when I was a kid. I would drive to Tucker Trap in southern Maine and buy 100 or so "trap kits." The kit contained all the materials needed to build the traps. I built them in my parents basement, then in the garage, and then, as I got older, in a locker at the "Fish House" in Swampscott, Mass. If I remember correctly each trap cost about $12.00 fully rigged.
The lower, wire trap, was one of the last batch of lobster traps I bought in 2000. I kept this one for old time sake when I sold the business in 2004. I added the rope bridal and the extra escape hatch on the top.
I've seen underwater videos showing young lobsters hanging around the trap - going in and out through the escape hatch to feed on the bait. The adult lobsters, over 7 years old, have to enter through the kitchen (the front of the trap), grab some bait, and come back into the trap (called the parlor) where they can't escape because they're too big to fit through the escape hatches. A trap like this one will last about 7-8 years of seasonal fishing - longer if left in the water year round.
When I bought the last batch of 100 wire traps they were about $52.00 each. Then I need 15 fathoms of rope, a buoy and a bait bag (shown laying in the parlor.)
I have a lobstering video on my website, shot by a PBS crew, and used as part of a sustainable seafood festival. It shows me in the early 1990's hauling my traps near Marblehead, Mass. - it's a download from this site but it's not free. Sorry, I have to pay for the download service.
If you're curious about lobstering you'll enjoy the video.