Asking 25k with only 21k miles sound about right?
I paid a lot less than that, but the Pro 4X does cost quite a bit more than the S. Once I decided I wanted an Xterra, I shopped around and spent a good deal of time before I found a great deal. I bought mine as a "certified used" from Torre Nissan in La Qunta. I negotiated back and forth via email for a few days, because I told them I was only making the 160 mile round-trip drive one time and I wasn't afraid to leave the dealership in my old car if they tried to change things up on me.
I bought it just about a year ago and it's a 2010, so it was 4 years old, with about 34,000 miles on it. In great shape with only one original owner (who always serviced it at the dealership, per the CarFax report), but it did spend it's whole life out there in the desert so the steering wheel had some sun damage and the plastics on the exterior were sun faded.
They were asking $19,900. In the end, I got them down to $18,000 including a 7 year, 100,000 warranty. They also gave me a good value for my trade, which they hadn't even seen. I was pleased with the dealership and would do business with them again.