Brain Stuff have done a nice job of explaining the cost of running a campaign.
More resources for teaching and learning about the 2016 U.S. Presidential election:
A large portion of the money that candidates raise will be spent on television advertising. The Living Room Candidate offers a history of campaign commercials dating back to the 1950's. Students can watch old commercials and read the transcripts of those commercials.
PBS Election Central is a collection of educational resources related to the 2016 U.S. Presidential election. The collection includes interactive maps, virtual field trips, and videos. Within the PBS Election Central collection there are resources appropriate for elementary school, middle school, and high students.
C-SPAN's 2016 Electoral College Map shows the number of Electoral votes each state has in 2016. The poster also includes the Electoral vote and popular vote tallies of the every election dating back to 1900. You can get a copy of the poster here. The poster on its own is nice, but students will need some guidance in understanding what all of the information really means. To that end C-SPAN Classroom offers a set of guiding questions to use with your students in conjunction with the 2016 Electoral College Map.
This TED-Ed lesson offers a short explanation of the Electoral College by answering the question, "does your vote count?" The video for the lesson is embedded below.
Common Craft offers The Electoral College in Plain English.