1. Can we use Essbase with SQL Server BI?

Yes. Here is a good whitepaper on Essbase with SSRS: http://download.microsoft.com/download/4/7/a/47a548b9-249e-484c-abd7-29f31282b04d/UsingSSRSandESSbase.doc

Essbase does have an XML/A adapter. XML/A or XML for Analysis, is the XML specification that is used by both Essbase and Microsoft.

Both OLAP databases use MDX as the query language. But you cannot use PPS with Essbase.

2. Are there trial versions available for PPS?

I believe that this is good for 180 days: http://www.microsoft.com/business/performancepoint/howtobuy/trial-software.aspx 

3. How is PPS priced? http://www.microsoft.com/business/performancepoint/howtobuy/default.aspx