I see A LOT of single hungs being sold in my area, but only for new construction, and pretty much only for condos and apartments. So yes there IS a big market for them (but it's painful for me to see . . .) . It's usually a builder-driven decision, since most homeowners would have selected a double hung. You can normally save some money by going to single hungs (in new construction), but almost everyone likes double hungs better. Easier to clean. Sometimes believe it or not the single hungs perform better than their double hung counterparts, but that's not the primary reason that single hungs are used. It's usually about price. And some are made pretty cheap. I agree with Windowrep - you'll no doubt be glad you selected DH's instead of SH's. One warning - if you mix single hungs and double hungs, they may not match in appearance. The profiles may look different, and single hungs may have half screens while double hungs may have full screens.
As for grilles, they usually cost about $3 to $6 per lite, depending on the brand and markup, so a double hung with 6 lites per sash would add up to 12 lites x $3 = $36 or more. Get the grilles from Windowrep, his are only $15-$25.