This is as cheap as they will ever get. All versions listed come with all the DLCs. 3.40 Euro = $3.60 US = ~ $5 AU. They are $6.50 US on sale at Steam in comparison.
Fallout Vegas Ultimate: https://de.gamesplanet.com/game/fallout-new-vegas-ultimate-e…
Elder Scrolls IV Oblivion GOTY: https://de.gamesplanet.com/game/the-elder-scrolls-iv-oblivio…
Fallout 3 GOTY: https://de.gamesplanet.com/game/fallout-3-game-of-the-year-e…
Morrowind GOTY: https://de.gamesplanet.com/game/the-elder-scrolls-morrowind-…
GoG also now has a DRM free version of Morrowind (preferrable if you want to mod it), but it isn't on sale.
Update: The base Skyrim game is 3.75 Euro https://de.gamesplanet.com/game/the-elder-scrolls-v-skyrim-s…
Skyrim Legendary Edition (which you really need if you want to mod since so many esps have DLC requirements) is considerably dearer at 10 Euro, almost on a par with Steam's pricing: https://de.gamesplanet.com/game/the-elder-scrolls-v-skyrim-l…
Rage: 2.50€ https://de.gamesplanet.com/game/rage-steam-key--1056-1
Doom 3 BFG Edition: 2.50€ https://de.gamesplanet.com/game/doom-3-bfg-edition-steam-key…
Hunted: 2.50 € https://de.gamesplanet.com/game/hunted-die-schmiede-der-fins…
Skyrim, FNV and FO3 have seen lower prices on Steam. Might be worth holding out till December sales if looking to buy exclusively off Steam.