According to https://en.wikipedia.org/wiki/Mosquito_laser it would cost $50. They deliberately built it using already available parts (the laser is the same as used for bluray for example) to keep cost down. The problem seems to be (as parent pointed out) reliable electricity.
If I'm remembering the conversation correctly, the most expensive component was the kill laser (it uses a lower-power laser to track flying things & match the mosquito size / wing frequency and then fired up one strong enough to fry an insect) but that hopefully has been going down since those figures were calculated years ago.