I do my own so it's hard to beat "free". Are you still qualified for Toyota's free maintenance program?
I had it done recently at Discount Tire, and there was no charge. Great Price! I had purchased tires at Discount for my previous '02 Prius, but not for the '08 I have now. Maybe that is why they didn't charge me??? In the past Belle Tire has fixed leaking tires for me with no charge. I think that these tire stores offer freebies in the hope that you will come back to buy tires & other services.
I think the OP is posting in the wrong part of the forum (a 2010 is listed as the vehicle), but he/she is welcome anyway. As posted above, many tire shops will do it for little or no money in the hope you will come back and buy tires (Kal Tire here calls it "true service"). And I do buy my tires from them for this and other reasons. If you are charged it should be in the $25-$50 range. Especially for a Prius. It's so easy (the wheel/tire combo is so light and small).
Usually, any place you've purchased tires or had several services done by will do it as a freebie. There are places here, that decided to charge $5/tire if you didn't purchase the tires there. The local Pepboys (which I wouldn't use for anything other than simple, basic tasks) will do it free because they mounted & balanced my last tires. They said that a rotation would even include swapping my winter tires/summer tires whenever I wanted that done. Personally, if I have the time, I'll do it myself. It might take 45 minutes taking your time, getting tools out, moving car (if necessary), etc, etc. I usually wait at least that long anytime I walk into a shop to get anything done.
Instead of rotating I buy new tires in pairs. When the front two wear out the new ones go on the rear. This increases value extracted from each tire, maximizes safety, and minimizes bother.
I saw what the Toyota dealer wanted for a 15k mile rotation and balance - around $50. So I went to Discount Tire. For $55, they sell a service contract that gives you unlimited rotations and balances and a road hazard warranty (for the OEM Integrities, prorated for mileage) for 55k miles. Now I get the tires rotated and balanced every 5k miles - for free.
I second all of these. It shouldn't cost much and if you buy tires from tire stores like America's Tire they'll give you free rotations. I had to buy only 1 Bridgestone tire 2 years ago and now Firestone gives me free rotations for all 4 (though I think it's because all 4 tires are bridgestone).