Red tail giant gourami I think are more expensive than GG's, Wildwoods is offering them for sale at 5" for £60 (but wildwoods is rather expensive compared to other shops so I would say anywhere from £40-70 depending on size and quality). Which really isn't alot when you think of how long the fish will live and the entertainment you'll get from it. The real cost comes from what is needed to keep the fish eg. tank, heaters, light, filter, and with all big fish - food - and lots of it.
On the oscar question I got 1 tiger oscar (always had that kind). My favourite type though is the lutino.