A sequencing run (at least in the context of Illumina's technology) requires a few very expensive consumable reagents: First being the flowcell (microscope slide that the dna sticks to while being read by a laser), the reagents (containing enzymes with fluorophores and other reagents for amplifying and manipulating DNA), and the actual power consumed by such machinery. This does not factor in prep/lab costs (which can be kept at a minimum with automation, but that also is a high startup cost endeavor). Each sequencing run can take ~1-3 days depending on the format.

Edit: this video may be able to explain a little better how this process works: https://www.youtube.com/watch?v=fCd6B5HRaZ8