C:\Windows\Microsoft.NET\Framework\v4.0.30319\aspnet_regiis.exe -i
Windows Performance Monitor should be able to get you pretty close to what you want. There are literally hundreds of metrics in there to use.
To access it, simply do Start -> Run -> perfmon From there, select 'Performance Monitor' in the left pane, and click the '+' button to begin adding in counters.
If its an ASP.NET based web site, you can select one of the 'ASP.NET Apps' counter categories (may have more than 1 if you have more than 1 version of ASP.NET installed) and click on the 'Managed Memory Used' counter. In the list below that, all actibe websites are displayed and you can add the counter for that. You can watch the counter in realtime or elect to save the data to disk or a DB for later analysis.
There is a W3SVC_W3WP counter category that allows you to examine metrics for specific app pools but nothing (that I can see) that will offer memory used per app pool.
Depending on what you are trying to determine though, you may be able to find a bunch of metrics to aid in your analysis