to my opinion here are some factors that contributed to decrease sequencing costs (some were already mentioned). Note that not all points can be applied to all technologies.
- Parallelization of sequencing reactions. Technologies such as Illumina, IonTorrent or Nanopore use technologies that can easily be parallelized (clusters of oligos, microchip and nanopores, respectively). This allows to run a large number of parallel sequencing reactions in a relatively small instrument. With Sanger sequencing, to parallelize reactions you need a huge amount of individual sequencers.
- Less expensive chemistry. Most of new technologies (e.g. IonTorrent, Nanopore, 454) sequencing libraries do not rely on modified nucleotides (ddNTPs) that are actually expensive. I don't have any numbers to provide you with, but I guess this can be found online. This also (sometimes) implies shorter protocols for libraries preparation which therefore decrease the human cost. Some technologies such as Oxford Nanopore Technologies (platforms are not commercialized yet) do not even need some expensive chemistry since they are mostly relying on the electric signature of nucleotide interacting with the nanopore. In general, excluding the presence of ddNTPs and/or dNTPs considerably reduces the cost of sequencing.
- Increase in throughput. The decrease in sequencing cost is tightly linked to the increase in sequencing throughput of the "next generation" sequencers. In one single run (that still can take days or weeks) one machine can sequence several gigabases of DNA while previous technologies (i.e. Sanger-based methods) were more in the order on megabases.
- Absence of fluorescent signal detection. Sanger, Illumina or 454 technologies need to detect fluorophore signal to know which nucleotide was inserted during the sequencing reaction. Lasers and scanners that fulfill the accuracy requirements are very expensive. That, in part, explains the discrepancy of prices between an Illumina (fluorophore-based reaction) machine (~500K$) and plaforms such as IonTorrent (from ~50k$ per machine- microchips detect nucleotide incorporated) or ONT platforms (announced price of ~30k$ per cluster - nucleotide detection also rely on electric signal detection).
Some data come from this comparative table of sequencing technologies
Disclaimer: I didn't mention technologies such as Pacific Biosciences or Helicos since I am not familiar/documented with these technologies.