[The following is specific to my AD&D South Province campaign, written in May 2004, and has no real world use. Some of it might be useful for other Dungeon Masters.]
Social class should be having an increasing affect on PC-NPC interaction as the game progresses. I have provided here my number-crunching-related thoughts on social class advancement.
Maximum social class is dependent on level. In this region, it works as follows:
0 level: LLC-LMC
1st level: MMC
2nd level: UMC
3rd level: LUC
4th-7th level: MUC
8+ level: UUC
Social class can be advanced by in-game behavior, such as Alex being "knighted." Also, it will automatically advance as PCs advance in levels. Higher level PCs will just have an aura of being important about them.
Clerics: +1/4 levels
Druids: +1/5 levels
Fighters: +1/4 levels
Paladins/Templars: +1/1 levels
Rangers: +1/3 levels
Magic-Users: +1/2 levels
Illusionists: +1/3 levels
Thieves: +1/7 levels
Yes, disguise can, on a successful CHA check, conceal social class.
The price for a slave will vary. A disreputable lord might sell his excess serfs for as little as 10 gp each if he needs the money. A peasant might sell a child into slavery for 10-25 gp to feed the rest of his family. Someone in debt can be bought into slavery by whoever pays off his debts, however high those may be.
Slaves will turn up in any of the minor marketplaces of Hexpools (by the North or South Gates, Fountain Square) when supply is available. There is no steady supply of slaves, so demand is never fully met.
Usury exists, and is actively encouraged by the Zilchites. Interest on loans will vary from 1% per month from a reputable moneylender to 10% per month from a thief.
The hierarchical ranking of titles in South Province is:
__________Upper Upper Class
Freiherr (baron - larger holdings)
Waldgraf (baron - smaller holdings)
__________Middle Upper Class
Rheingraf (baronet - larger holdings)
Burggraf (baronet - smaller holdings)
__________Lower Upper Class