I'm still not sure what my price limit is for G+... but I actually I know I'm not there yet.
Currently my family is just my wife, my <3 year old son, and myself.
My last Disney trip... for 2 park days I spent $417.11 a day on tickets (park hoppers), $315.85 per night at the Swan, $75 per day on photo pass, and averaged $131.40 a day on food and drink. So for 2 people (son was "free"), I'm spending $939.36 to enjoy a day in the park ($447.25 on non-park days). This doesn't even include the plane tickets or transportation to WDW from MCO. My son gets up early allowing us to make an 8:30am rope drop, but we have to leave the parks by 7:30pm to get him to bed. Meaning I have ~11 hours of park time a day. Every hour I am in the park, it costs my family ~$85.40 or $1.42 a minute.
Waiting in line is "wasted" time in my day. In order to be worth it to me is the cost of G+ it must save my family: The cost of G+/$1.42 to be "worth it".
If Genie plus is $40 per person (total of 80), 80/1.42= 56 minutes saved... A single LL for many of the big name rides can easily save 56 minutes, I usually get at least 2-3 "big" rides a day.
As awful as it sounds... $40 per person is only ~8.5% increase on the price of my park day... If 8.5% less of my day is spent in lines...
I wish this wasn't the system we had... but this system is still worth it for me.