We really like the QSDP. We used it on our most recent trip (2 adults, 1 child & an infant) and also last May (3 adults, 1 child & an infant). In both cases, it definitely saved us money. One thing to think about--if you are doing more than 1 sit-down meal per 3 or 4 days (for you, 2 TS or Character meals during your stay) in addition to the QSDP, you are probably not going to see much of a savings and you are going to have a hard time using all of your QSDP credits.
I think it is $11.99/child per night and $3X.99/adult per night...I can't remember exactly. I plugged some dates in the Disney website and got $376.84 if you are staying 8 nights and $328.86 if you are staying 7 nights. The price is based on the # of nights you are booked at the resort.
Your estimate of $340 seems low for 1 adult/1 child for 8 days. That averages out to slightly more than $40 per day. Did you include taxes and snacks? That's what I like about the QSDP--it includes the sales tax and two snacks per day per person.
Hope this helps!