How does the cost of designer board games compare to other forms of entertainment?
I'm an aging grog... For me, I judge a game based upon player-hours of play. At about $1 per player-hour. Same rate I've used since the late 80's...

And at that rate, the $30 supplement I just got for TTR/TTRE has some way to go. 1st two plays were 4p @ 2.5h and 3p @ 3h - so far, $19 of the $30. One more large play should do it.

Dominion and Intrigue got enough time to "pay off"... and did so rapidly, even tho most were 2p games. 

The $50 Cargo Noir is close. At 1.5 to 2h per play, and 3-4p per play, I've gotten 7 plays out of it since mid Dec.

Now, I will say, I never adjusted for inflation, and find most movies on DVD today not worth the $1/person/hour, so that metric still works for me.

You might, however, consider a more movie based metric. A movie is $5 at matinee, per person, and typically 1.5 to 2 hours.  If we add the anticipation and post-movie discussions, we can justify calling them 3 hours. That calls for $2 per person-hour.

I'll note also that I'm 56 hours in on a $50 videogame (Skyward Sword) and 30 in on another (Ocarina of Time 3D), so that $1-$2 per player-hour is quite well holding. Mario-Cart, at that rate, paid off in the first week: 10 hours, 3 players, two days; $20 paid for the disk.

Update for Dec 9 2012: The latest supplement I got for TTR paid off with another $12 of play.

Update for July 2013: All the mentioned games have since paid off. And the Wii has paid off.