The Windows Server "8" Beta is free for evaluation.
Microsoft has not yet announced the final release dates, including SKUs and pricing for Windows Server 2012.
As you asked for comparison, Windows Server 2008 R2 pricing varies by SKU and VL agreements. However, a good starting point is that the retail box of Windows Server 2008 R2 Standad w/ 5 CAL is about $900 USD.