Manga titles have known this for years. It's the price of the first Chew trade paperback. Vertigo have had a spate of them, Crossing Midnight, Unknown Soldier, Scalped and now Dynamite's Project Superheroes is using the price point for their new trade.
$9.99 is a golden price point for trade paperbacks to entice the unsure reader, to let them gamble on an unknown property and get them hooked for trades yet to come. And the Project Superpowers: Chapter Two TPB fits seven issues (0 to 6) at that price, 180 pages, 5 cents a page for you number crunching fans.
Of course they're not forgetting the sales of regular single copies either. Just as with Chew #1, the reduced price trade paperback ships on the same day as #7 of the series appears, giving retailers the opportunity to upsell on the trade paperback to add a copy of the seventh issue to the pile as well.
Basically you're getting a trade, consisting of 5,6 or 7 issues for the price of two Avengers annuals. And from all accounts they seem to work, they attract people to a series, they provide an affordabnle jumping on point and, in comparison with other books on the market, they're damn good value for money too.
$9.99! It's the future!