Tricky one - I didn't bother with the service plan on my last audi (TTRS+) and ended up paying £300 plus change on the first service, then £690 on the second. Service Plan was the long life as I was doing 20k a year. So, a grand in two and a half years / 44k miles. The amount I was quoted by Audi, paid monthly, would have equated to £1650 to the point where I sold the car. So, do the math.
Having said that, on the S3 SB I had, I did 36k miles, two years of ownership. Service Plan I had was just £35 a month based on 10k a year, which was £840 for total length of ownership - would I have paid more without the Plan? I think it would have been close.
For me it's not so much a cost saving, it's more a decision on how you manage your finances. Right now I can afford to take the hit of a service bill as all the wife needs to do is put in some overtime (she's a GP) LOL! But a few years back, when the missis was unwell and the company I was managing had a lot of problems, I preferred to have a budgeted value.