Here is a version comparison
In Australia, CS5 shows as costing $1168, CS5 Extended costs $1761
However, as you are looking at the Student/Teacher edition, CS5 extended costs $201.00
The reason you are seeing a price difference is that you are comparing the regular price for CS5 with the Student price for CS5 Extended. There is no Student version of just CS5
Thus, if you are student you are better off buying the CS5 Extended, even if you do not plan on using the 3D features.
Here is the info on the usage limitations for academic editions;
In terms of usage, all I could find was that it must be for use on your personally owned computer. I can't find anywhere that you are not allowed to use it for commercial work - just not on a commercially owned computer. For example, if you worked from home for a photography company, and they provided you with a PC, then you would not be able to. However, from what I can read, if you worked from home for a photography company and used your own personal pc, then you are eligible. For example, the blurb on the Student/Teacher edition of Litghtroom 3 tells you to;
Focus on what you love about photography with Adobe® Photoshop® Lightroom® 3 software, an intuitive digital darkroom and efficient assistant designed for serious amateur and professional photographers
I would assume a professional is one that makes money from their photos...
Quoted from Adobe's Restrictions of Use;
Restrictions on useIf in doubt you could always contact Adobe.
You may purchase only one license of any Adobe Student and Teacher Edition product within a 12 month period. Purchases of Suite editions (such as Design Premium or Master Collection) are limited to one title per platform per year. This license may be used only on your privately owned computer. Student and Teacher Edition products may not be resold
Hope this helps!