Assuming:
You get nothing from your trade (i.e. they just pay off the balance)
The Prius costs you $26,500 out the door (incl TTL, etc.)
Gas is $4 a gallon
You drive 12,000 miles a year

Buy a used car.

Even if you buy a used Suburban--which people are dumping in droves--the cost savings up front make up for the better mileage. (And I'm not even calc the additional insurance on a new car vs. a used one!)

Let say you pick a Suburban up for $15,000 and get 15 mpg. It'll take over 5 years and almost 63,000 miles to make up for the extra $11,500 you pay for the Prius. That's best case (i.e. you pay cash and have no interest on a car loan.) Here's the math:

Suburban: 15mpg = .067 gal/mile. .067 x 12,000 = 804 gal/yr x $4 = $3216 yr
Prius: 48mpg = .021gal per mile. .021 x 12,000 = 252gal/yr x $4 = $1008 yr

Prius saves $2208 a year in fuel. $11,500/$2208 = 5.2 years

If you finance the car:

$26,500 for 60 moths at 5% = $500.09 payment
$15,000 for 60 months at 7% (used car higher rate) = $297.02 payment

That's a difference of $203.07, or $2436.84 a year, $228 more than the $2208 a year you save in gas with the Prius.

This is the worst-case Suburban. Buy something with better mileage and the numbers get even better.