Currently, you would need a VBP license for each computer where builds will be performed, although we are looking into offering a runtime version of VBP for a future release (but it will probably not be free).
For InstallShield, I believe that you need the Premier edition (which costs $4,000+ for one developer) in order to install and use their stand-alone build tool on up to 10 other computers (see
http://community.installshield.com/s...d.php?t=178625 and
http://www.acresso.com/webdocuments/..._IS2009_en.pdf). Compared to that, VBP is already much less expensive to just purchase full licenses or a site license.