Unless you're getting it free, it will always be cheaper to pay out of pocket. We had free QSDP this year and I upgraded us to the
DDP - purely because it was just the two of us for only 7 days: I wouldn't be able to afford it otherwise. You have to pay the tips for the TS meals anyway, so it can still work out fairly expensive.
Our last 7 trips we did it purely paying for all our meals ourselves, and ate mostly off Disney property. You can get great deals at places like Golden Corral, Outbacks and even Dennys for dinner - we usually average approximately $14 per person including the drink.
It is very rare to find any deals for free dining during busy holiday periods: there's no incentive to try and get people in during those times, you see. Indeed, it's getting more common for Disney to offer discounts on the rooms rather than 'free' dining, and unless you have a full room (ie 3 or more people) it doesn't work out well economically for the dining anyway. I've actually PAID for our QSDP for this coming March, but only because I got TWO flights out of the three booked completely free!
You also have to buy the most expensive Disney tickets in order to even qualify for any dining plan, which can work out a lot more money if, like us, you're used to buying the cheapest 5-day MYW base tickets from the US sites.
So, my base line answer is: don't worry about trying to get the dining plan. If your room was cheaper as a result (which it usually is), then it'll cost you a lot less at the end of the day.
