NV is basically a port of Fallout 3 so your paying a new game price for a conversion of an old game.
What? That makes no sense, not to mention the argument you're trying to make is so annoyingly drole and passť. So what if FO:NV is similar to FO3? Was FO2 not similar to FO1? Was BG2 not similar to BG1? Why does it matter so much if a sequel is similar to the game that precedes it? Heck, wasn't Beyond Divinity (To use a Larian example) somewhat similar to Divine Divinity?
It's not a "port" as it's not been transferred from one platform to another. It's not a conversion as they've not taken FO3 and tweaked it. It's a standalone game that overhauled the mechanics of FO3. S.P.E.C.I.A.L. was updated, the perk system was overhauled, traits were added, it's a whole new setting (Sort of, anyway) with new gameplay mechanics and new items and characters. It's much more than a conversion.
It was released in a more bug ridden state than most Bethesda games and that takes some doing.
And? Bugginess seems to only count against certain games. BG2 and NWN were famed for being quite buggy, too. Yeah, NV is buggy but it's no buggier than a number of other games on the market. At least Bethesda and Obsidian are working to fix the bugs, unlike some developers/publishers.
It uses an old engine that Bethesda never seemed to master, (even though the guys from Larian can make it hum along quite nicely).
Bethesda are the only studio to really have cocked up Gamebryo to this degree. However, New Vegas (for me) is by far the most stable iteration of Bethesda's modified Gamebryo to date. FO3 blue screened constantly with my drivers and modding constantly broke Oblivion for me (Not to mention it kept falling over anyway).
Its a Steam activated title, might not put you off but it does put others off.
Forced patches from Steam breaks everybodies mods.
Not everyone mods the game, and I don't think Steam has any factor on sales at all. Civ V, Half-Life 2 + Episodes, Portal 1, TF2, Dawn of War 2 + Chaos Rising, MW2, Black Ops (I think that requires Steam, could be wrong) - All have done, in terms of the PC, incredibly well.
And now, so soon after release, its gone cheap, in fact, its half price.
Which is perfectly normal for the UK game market bar Activision titles. A lot of big releases drop massively in price after a short while - I've seen Dead Rising 2 at £15 for the PC.