It varies greatly, and the figures generated by third party studies are almost always lower than the self-reported figures. This article [1] from 2012 reports per-drug pharmaceutical R&D spend ranging from $3.7B to $11.8B. It's not the most transparent cost analysis.

[1]: https://www.forbes.com/sites/matthewherper/2012/02/10/the-tr...

Depending on what timeframe you're talking about for that change, the costs have in fact gone up, both because low-hanging fruit has been picked and because the regulatory regime has been changing over time to required more and more work to bring a drug to market.