The US ticket onsales occured in the final week of september. This was after the Rehearsal gig, after the Riverside gig and after a handful of TV gigs. Word was out that Bowie was dipping generously into back catalogue. Word was also out that he had done so on the 99 tour, 2000 dates, Tibet appearances, Heathen Appearances, Marathon shows etc. Also keep in mind that a week after the US onsales, the tour opened in europe, with full knowledge of songs spread around the net.
One specific example that needs looking at is Vancouver. The initial onsale was for the "theater/1/4" stage set-up. Then actual show occured almost in a full set-up. In the 3 months between that time the stage was bumped back 2-3 times. Documentation of this happening has appeared on this site. I would safely say that attendance was DOUBLED from first months sales to showtime. That would be a 100% increase. Back in september or April I said the 4 Canadian dates would all do over 10K(Calgary 13K, Van 12K, Mon 11-12K, TO over 10K) and I was laughed at by some folks here.
You can tell that most were there to see the hits. You can judge this when by the reaction to the Reality material(noticable dips of interest in BC, strong reation in Seattle) That said, I was quite impressed by the reaction to the Heathen material and couple songs form the mid-90's.
The recent tour simply illustrates that strong attendance numbers will be drawn for Bowie's "proven" material. By proven I mean both songs that work well at all the NYC or europe shows. Some "hits" get tiresome and some newer stuff receives huge interest, and it changes from tour to tour. Large US audiences don't seem to enjoy being the full-on guinea pigs for Bowie's creative indulgences.
PS Bowie mainstream shows have a high "walk-up" number, so the shows in april should do decently come showtime.