Well, I found a couple things by delving into the voluminous user submission agreements:
“Transaction Fee” means a percentage of Net Sales of the Game retained by Microsoft as a fee for distribution of the Game. Using automated tools, certain games will receive prominent placement on Xbox LIVE Marketplace or will otherwise receive marketing and promotional support from Microsoft (a “Featured Game”). The Transaction Fee for each download of the Game sold while it is a Featured Game will be at Microsoft's sole discretion, up to a maximum of 60% depending on the level of marketing and promotional support as further detailed in the Xbox LIVE Indie Games Guide. The Transaction Fee for each download of a Game other than while it is a Featured Game will be 30%.
4.5 Statement. After the end of each calendar quarter with respect to which Microsoft owes you at least USD $150, Microsoft will provide you with a statement and the amount owed. If Microsoft does not owe you at least USD $150, Microsoft may carry forward the amount owed until such time as Microsoft owes you at least USD $150. In the event Microsoft ceases distributing your Game and owes you less than USD $150, you agree that, as a result of the cost involved in payment processing for such small amounts, you waive your right to payment of this balance.
So, the payments are in increments of $150, but the fees can be up to 60%, solely at their discretion.
I had to read that several times.
Oh, and the 'review' process is a peer-review from other devs.