You can get some info on the I/O Vendor Partner program here - https://www.vmdev.net/sf/sfmain/do/home. You can also download the driver source code to get started - http://downloads.vmware.com/d/info/datacenter_downloads/vmware_vsphere_4/4_0#open_source
and welcome to the forums!
Please note that you are replying to a thread that is about 7 years old now. It is unlikely that this will get much attention now. Anyway I will try to answer your question.
The tools and - more important: - the source code you need will probably be provided for free, but as a pre-requisite you need to join the "SDDC Integration Program" which in turn requires a "TAP Elite Membership".
According to information available here:
both memberships cost 7.500 US$ per year, so 15.000 US$/year in total.