Pretty self-explanatory. Either you have the PLE, or you pay $4000+
Pricing and Availability
Autodesk MotionBuilder 7.5 is now available to MotionBuilder Platinum Members. The software is expected to become available worldwide in late March 2006. Autodesk MotionBuilder 7.5 suggested retail pricing (SRP), including one year of Platinum Membership, remains unchanged from version 7. Autodesk MotionBuilder 7.5 (node-locked) SRP is US $4,195 * and Autodesk MotionBuilder 7.5 (floating) SRP is US $4,795 * in North America. MotionBuilder software is available for both the Windows and Mac OS X platforms.
Note, there is no PRO or STD, only PLE or ‘Motionbuilder’. Also note how they cleverly avoid mentioning Std vs PRO in the FAQ, again only PRO vs PLE. Oops, sorry - my bad ‘Motionbuilder’ vs PLE. 

