$500k for 1800 lights is $277 per fixture. That's the price range of low/mid-level DJ lighting, just to purchase, not to install.
I found the manufacturer of the fixtures that were installed:
The gateway to Tampa Bay is the four-mile long (6.7km), 430 feet (131m) high, Sunshine Skyway Bridge. This flagship bridge has undergone the installation of...
www.lumenpulse.com
These are IP66 rated (i.e. waterproof against strong jets of water, but not immersion) and several times brighter than a basic dj light.
The contract price of $15.6m comes out to $8,667 per fixture. What little I found of pricing on these lights suggests something in the range of $1-3000 per fixture, depending on options, volume, markup, etc. Then there's the required engineering design and review, custom mounting hardware, electrical and control systems, multiple miles of cabling, and a whole ton of labor.
The construction company that won the bid has a bunch of pictures of the job on their facebook page:
M & J Construction, Tarpon Springs, Florida. 102 dáma. We are a Florida Construction Company based in Tarpon Springs. We specialize in DOT contracting and coatings.
www.facebook.com
It does not surprise me at all that it cost several thousand dollars to install each light on the side of a very busy bridge.