Is there a baseline value for how much RAM an app pool consumes under IIS7?
An app pool just serving up static content is less than 5Mb. Once you add the .NET framework then it will jump to over 10MB (it really varies on what is loaded). After that it's up to your application as to what it uses.
If you're wondering how much app pool overhead there is to put sites in their own app pools, it's really not much. There is the small bit of app pool overhead, and it does mean that your programming framework (i.e. asp.net) may have duplicate assemblies loaded into memory. That means that there may be as much as 15-20MB overhead per additional app pool. With memory so cheap, it's often worth it, although it can add up if you have a ton of sites.