[Reasonable price?]

I bought mine almost a decade ago for about 100$ on ebay (a loose one).  Still working with 8 games, a few manuals and overlay. However, the controller itself has one of its button "semi-stuck" but still work well.

[Which is the best Atari 7800 emulator?]

Bizhawk is my favorite emulator when it come to the Atari 7800, alot of progress was made to support the commercial homebrew games.

[Klax the last 2600 release.]

As far as I know, Acid Drop was made the same year that the Atari 2600 production's was over, in 1992. Klax is the final release published by Atari thought.

[Your Most Difficult 2600 Game?]

Got to say, Indiana Jones is the cryptic game of the batch.