A rough ballpark estimate of the costs of an inter partes review is approximately $300K to $600K. That is by no means cheap, but may still be considered a bargain compared to the costs of defending a patent infringement litigation in federal court which could easily reach $1-4 million or more. If an IPR is instituted, USPTO fees alone start at $23,000.
In view of these six-figure costs, it makes a great deal of sense to invest in thorough prior art searching.
Federal court judges have generally been favorable to staying, or suspending, a pending patent lawsuit to wait for the outcome an IPR which concludes within 1 year from the date of institution. Therefore, patent infringement defendants can defer hefty litigation costs, particularly those associated with litigation discovery, while contesting the validity of the patent-in-suit before the Patent Trial and Appeal Board (PTAB) at the USPTO.