The Holiday Inn off the M4's junction 32 has upped its price from £97 to just under £600 for the night of Ireland's game against Canada.
The nearby Village Urban Resort has rooms for £499, up from £105.
Cardiff's Hoteliers Association said the prices were "in tune with demand".
Jonathan Jones, former chief executive at Visit Wales, warned accommodation providers' high prices could prevent return business.
He said: "If all hotels and B&Bs  in Cardiff put up their prices to that extent, then it could seriously damage the image of Cardiff.
"I would sincerely hope that they remember that they need to have these customers coming back, and will not simply see this as one instance where they can make a lot of money, over a very short period of time."
Not all hotels are cashing in to the same extent.
Carl Davies, general manager at Jolyons, a small hotel near the Millennium Stadium, said it had only slightly upped its prices to cover extra costs.
He said: "We have to extend our service hours and we have to have more staff in. We only charge around £60 more than our usual room rate - purely because we want repeat business."
Hotels increasing their prices six-fold during the Rugby World Cup could "seriously damage the image of Cardiff", a former tourism chief has warned.