I'm having trouble understanding the example of rule 602.1h in the Magic Comprehensive Rules.
601.2h The player pays the total cost in any order. Partial payments are not allowed. Unpayable costs can’t be paid.
Example: You cast Altar’s Reap, which costs {1}{B} and has an additional cost of sacrificing a creature. You sacrifice Thunderscape Familiar, whose effect makes your black spells cost {1} less to cast. Because a spell’s total cost is “locked in” before payments are actually made, you pay {B}, not {1}{B}, even though you’re sacrificing the Familiar.
If the costs are locked in before the sacrifice lowers them, why does it say only {B} is payed instead of {1}{B}? Should it not be the other way around?
The language ("even though...") also seems to suggest the costs were mistakenly exchanged.