Hi Arie,
By road from Kew to Wisley is about 27 miles if you go via the M3 and M25 and takes about 35 minutes driving. I'm sorry but I have no idea what a taxi would cost for that distance/time. The alternative is train from Richmond to West Byfleet and then taxi from there to Wisley (about 4 miles so this should not be too expensive) - this is the route as suggested by Neil in the previous post. The only bus that comes to the garden is the one detailed also in previous posts - route 515.
Hope this helps a little
Paul