Looks good.
I'm a contractor and mainly do custom hardwood floors. You have to watch your floor guys to make sure they don't use cheap finish, especially when it's a polyurethane. Bona and Duraseal make good polyurethane finishes. Just about ANY other polyurethane is crap, especially any home depot products.
Since you have older 3/8" or maybe 1/2" flooring, and nails are starting to show through... you probably should have gone with a tougher finish. If it's too thin to sand again, you're FUBAR if you scratch, dent or crease them.
An acid based finish like Glitza or a moisture cured urethane would have been a better choice as they are far tougher finishes.
Though, if your floors are the newer 3/4" thick or they've still got a bit of their overall thickness left after sanding, Bona's "traffic" finish is by far the best out there or another waterbased finish that is fairly close is called Streetshoe.
$3 a foot is pretty good, I only cut people a break like that when either business is slow or the job is pretty large. The median price for sand and refinish is about $3.50 a SQ FT, but from a medium color of stain and darker the price goes up.