Does a secure erase on a SSD with lots of free space cost less P/E cycles than an SSD that's almost full?
It depends on how the secure erase is implemented.  If it just deletes the encryption keys without actually writing to storage, it doesn't cost any P/E cycles in either case.  If you go through and zero all blocks that haven't already been erased, then it depends which disk has more blocks that need to be erased.  That isn't straightforward to relate to free space on the disk, since blocks are not necessarily erased when freed.