Because it's a mid-engine car, basic service is more involved on these than most. That might address at least part of the reason the cost of maintenance is higher than one would generally expect. Also, if you haven't already, consider the probability you will own the car for longer than four years in order to take full advantage of prepaying. I would like to think I'll have mine for at least four years, but you never know. For that reason I passed on the four year prepaid plan, but I did prepay for the 2nd year service at a 25% savings to the service department's quote for the same that day.
I assume the $2800 you were quoted only covers the 2nd, 3rd, and 4th year service. Three services, not four, since as you mentioned the first service is included in the price of the car. At the same time, if I'm not mistaken, the 2nd and 4th year services (and every 2 and 4 years after) are more comprehensive than the 1st, 3rd, and so on which are more along the lines of an oil change. That is to say, I think you can expect even year services to be more expensive than odd. (The even year services differ in cost from one another as well, with the four quoted approx. twice the two.)