Well, the Prius is considered a "midsize" sedan getting 50 mpg. A Camry gets 28 mpg combined. Not sure if you has to wait ten years to realize the difference. A base Prius costs low $20k. And most Prius gets more than EPA especially if you drive city a lot.
Lets say average driver drives 12k miles a year and average gas price is $3.50 per gallon (averaging today and near future prices).
Average fuel economy for Camry (base) is 28 mpg (combined). Average fuel economy for Prius (base) is 50 mpg (combined). Both use regular gas.
After 12,000 miles, the Camry uses 428.6 gallons of gas. Prius uses 240 gallons.
In that time, Camry owner would spend $1500.01. Prius owner would spend $840. Difference of $660 per year (if driven 12k miles in that year).
Price difference b/w Camry and Prius is insignificant.
BTW, if you want to compare Corolla (a smaller & less practical car than Prius!), then Corolla gets similar MPGs as Camry 4-cyl. Lets say that a Corolla is $16k, while Prius is $22k. (Remember, Prius base has a lot more features and tech than a base Corolla.) But, for fun, lets do the same math.
Since both Corolla and Camry 4-cyl gets similar mileage, the calculations above is valid for Corolla as well. Difference is about $660 per year. Corolla is $6000 cheaper than Prius. $6000 divide by $660 = 9 years to make up the difference.
But, owning a Prius also means less maintenance than even a Corolla over the average lifetime. The ONLY thing that you do with a Prius is tire rotation, oil changes, and topping off accessory fluids (windshield, etc.). That's it. No tranny maintenance. No alternator to die. And as noted above (in Consumer Report testing), the brakes (and most other things) on a hybrid lasts forever! (The real brakes on a hybrid are used much less often.) Of course, the Prius is a much safer car than Corolla (see NHTSA) and has way more features (auto AC, one touch start, etc.)...and it has tons more room.
Disclaimer: i don't know about other hybrids...i only claim to know the Prius.