yes it is about money. to get dot approval they must meet certain specs and they charge for that. you just cant build something that meets those specs and put dot on them they have to be approved first
I'll say it again,
DOT does not "approve" anything. The DOT stamp means the builder believes (either by testing or whatever) that their product conforms to a federal motor vehicle safety standard (FMVSS).
This one for the lights actually.
FMVSS No. 108 Full Text
Here is a letter (concerning converting a dirt bike to be street legal but it covers the point I am making) http://www.bikersrights.com/nhtsa/notapproved/A-04.pdf
Scroll down to the yellow highlited text.
Long and short of the story, Just put some lights on, you'll be fine. If you can find a DOT mark on them, all the better..