I'm assuming that you have a 2006 Prius, and are in the US.
MSRP of the Toyota Prepaid maintenance plan, every 5,000 miles out to 4 years/55,000 miles: Classic (oil changes and Toyota Roadside Assistance): $540 Premium (Classic plan plus tire rotations, and major maintenance at the
15k, 30k, 45k intervals): $995 (includes 3 major services)
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Note that the MSRP can be haggled upon.
Also be sure that whatever prepaid plan your dealer offers is good at other Toyota dealers, and not just at that location or dealer chain.
In the US, the 2004-2006 Prius has a 5,000 mile/6 month service interval, which is an oil change AND a tire rotation. 30,000 miles is a much bigger service. 2006 Prius no longer comes with the complementary Toyota Roadside Assistance plan.
Depending on where you live (dealer labor rates and going rates for set services), and how good you are at remembering to bring your car in (the prepaid maintenance plan sends you reminder coupons), and especially if you can haggle a good deal on the Prepaid Maintenance, it may or may not be worth it to you.
Best bet is to call the service departments of several local dealerships, and ask what their going rate is on the Prius service intervals (be sure to ask for the "major" 15k intervals, too), and add those up. Also call the finance departments of those same dealerships and ask what the pricing is for the Toyota Prepaid Maintenance plan. For some people, the plan can be much cheaper than paying as you go, but for some others (depends on where you are) it is better to pay as you go...
If your dealer has forgotten to give you the Prius-specific Passport to Performance (scheduled maintenance guide), you can download it from:
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see exactly what the services include.