Icon and Screen Design for Mobile Device

For the development of an iPhone application we require the graphic design of various screens and icons:
1 Application icon:
Type: png
Size 57*57
1 AppStore icon should be .tif 512*512
Type: png
Size 512*512
5 to 7 iPhone Screens
Type: png
Size 320*480
The pricing should be structured by screens/icons instead of hours.
Upon successful completion of this project there might be follow up projects.