Windows Azure pricing is detailed here: http://www.microsoft.com/windowsazure/pricing/

There is a TCO calculation tool here: http://www.microsoft.com/windowsazure/tco/.

Some of the key cost differentiators, beyond just the hosting cost, include
- IT administration and support (including personnel)
- Licensing

It is also worth noting that moving to the cloud is often about more than cost. Examples:
1) Shifting CAPEX to OPEX.
2) Elasticity: Creating new business opportunities that would not have been possible without the elasticity of the cloud (e.g. products that need massive computation for short periods of time)
3) Geo-distribution: Many companies don’t have the means to do datacenter buildouts in all the parts of the world they would like to do business.
4) Time-to-market
5) Ecosystem benefits. E.g. Marketplace of standalone and buildling block services.