We replaced 18 double hungs last year for about $10,500. These were top rated UV windows, so we did qualify for the $1500 tax rebate. We used a local manufacturer because they are guaranteed for life, without having to deal with a middle man. If anything happens, the manufacturer comes out to fix them.
When we started looking, prices ranged from $10,000-$30,000. You really have to be careful pricing windows, because as PPs have stated, it's difficult to compare different brands and the prices are all over the place.