The search for the elusive Key Performance Indicators (KPIs) for eCommerce processes has been a challenge for as long as I've been around.
Austin Whitehead presented benchmarks at a recent Ariba conference and followed it up by listing them on the Ariba Exchange site. This is really good information. Note that in the External Benchmark column, there's a "1" prepended to the value (for example, for Processing Time for EDI Invoice the value of "1.0.25" is actually 0.25). Austin explains this in his comment below the chart. Take a look at his chart here.