Now, prepare yourselves... I'm about to do.... MATH!
This is a simple problem with a down payment of $99 at time = 0 following by a series of monthly, $15, annuities for 24 months. If we were given an interest rate, we could simply calculate the present worth of that series of payments, but alas, we do not have this so lets think about for a minute. What are we actually paying for here? We don't entirely know what this "subsidized" Xbox 360 package will include, but we can say that is might have a 4GB Xbox 360 (Best Buy Retail Price = $199). The 24 month plan should include Xbox Live Gold (Retail Price = $60/year, so for 24 months, that gives us $120 total value).
For this deal to be indeed a "deal" then the net present worth should be less than $199 + $120 = $319. We are still missing an interest rate. I went online and found common interest rates for credit cards to be around 20% APR, that's good enough. Unfortunately, APR = Annual Percentage Rate, not monthly percentage rate, so I need to convert 20% into an equivalent monthly interest rate. You may need to do a bit of research for this to make sense, but the equivalent monthly rate to an APR of 20% is... 1.2^(1/12) - 1 = 0.0153 or 1.53%.
Great, we have an interest rate, we have a payment plan. Let's NPW this shit. The present worth of a uniform series can be found here. I found the P/A for this = 19.96. Thus, the NPW of this payment plan is = $99 + (19.96)*($15) = $398.38
Well, you guys are right, this is not a "deal" per say, but it's not a terrible amount of interest, you are essentially paying $80 of interest over 2 years for the pleasure of enjoying an Xbox 360 with 2 years of Xbox Live. I've seen much worse payment plans out there for cars, dishwashers, couches, Pawn Stars...
Of course, I did not include tax or anything in this analysis. Hope that wasn't too long and boring. Of course, I'll try to answer questions.