This article in the Guardian in 2000 places DNA sequencing costs in 1985 at $10 per base location:
Fifteen years ago, decoding just one base letter cost $10 and was a painstaking manual task.
From an article written in 1999 it appears that the cost of DNA sequencing didn't fall at all in the late 1980s. However, since Craig Venter started using fairly new DNA sequencing machines at the NIH in the late 1980s I suspect there really was a fall in cost between 1985 and 1990.
When the HGP began in 1990, the cost was $10 per DNA base. The current figure is approximately 50 cents per base.By Randall Parker at 2002 September 05 09:59 AM Biotech Advance Rates