Can´t be precise, because I did those images with Artlantis R Beta, some 3 years ago.
As far as i remember, the house took half an hour or so (instead of 2 or 3 minutes), and the RIP CAD crashed because one of those trees didn't like radiosity (or the other way around).
On a wild guess, maybe 10 times more, with radiosity.
Bottom line, for the kind of work I do (quick and dirty, or at least dirty and dirty) it was too much trouble to insert the grass and render it, but for high end images, my guess is it is well worth the time you don´t have to spend photoshopping it later.