Sigil is a city run by and for the Factions, and life is easier in most quarters when a body's a member of one. Most services in the city (libraries, workshops, alchemical laboratories, craft guilds, healers, etc.) are dominated by the Factions. As such, they usually favor their own and charge outsiders a premium. Factioned PCs can seek out fellow members to keep prices low, but PCs without a Faction will often have to pay higher than the listed book price for certain services (GMs fiat). Regular market goods don't typically apply, so a PC buying a sword or a meal or a night at an inn will usually pay standard prices.

On the other hand, Factioned PCs (with the exception of Indeps) are subject to the hierarchy and politics of their Faction. When called upon to do a mission or serve a higher ranking member, they're expected to obey. And when dealing with the other Factions around Sigil, they need to be careful not to make friends (or enemies) with the wrong person.