What is the default Legend Cost game rule in CK3?

Normal: Legends will cost a normal amount. (patch 1.20.1.2)

Does changing the Legend Cost rule disable achievements?

No: none of its 5 settings carries the flag blocks_achievements in patch 1.20.1.2.

Game rule · Tweaks

Legend Cost

Normal · default
High
Very High
Low
Very Low

Legend Cost is a game rule in Crusader Kings III (Tweaks) with 5 settings: Normal (the default), High, Very High, Low and Very Low. None of its settings disables achievements. Patch 1.20.1.2, from game/common/game_rules/00_game_rules.txt.

This section as JSON: /raw/game-rules.json

Quick answers (2)

What is the default Legend Cost game rule in CK3?: Normal: Legends will cost a normal amount. (patch 1.20.1.2)
Does changing the Legend Cost rule disable achievements?: No: none of its 5 settings carries the flag blocks_achievements in patch 1.20.1.2.

5 settings

Normal
Default
Legends will cost a normal amount.
legend_cost_normal
High
Legends will cost twice as much as normal.
The script checks it 4 times, in 4 places
Script values 4
- legend_base_cost
- legend_game_rule_multiplier
- legend_piety_cost
- legend_prestige_cost
Every has_game_rule = legend_cost_high in game/common and game/events, by the top-level object it sits in.
legend_cost_high
Very High
Legends will cost four times as much as normal.
The script checks it 4 times, in 4 places
Script values 4
- legend_base_cost
- legend_game_rule_multiplier
- legend_piety_cost
- legend_prestige_cost
Every has_game_rule = legend_cost_very_high in game/common and game/events, by the top-level object it sits in.
legend_cost_very_high
Low
Legends will cost half as much as normal.
The script checks it 4 times, in 4 places
Script values 4
- legend_base_cost
- legend_game_rule_multiplier
- legend_piety_cost
- legend_prestige_cost
Every has_game_rule = legend_cost_low in game/common and game/events, by the top-level object it sits in.
legend_cost_low
Very Low
Legends will cost a quarter as much as normal.
The script checks it 4 times, in 4 places
Script values 4
- legend_base_cost
- legend_game_rule_multiplier
- legend_piety_cost
- legend_prestige_cost
Every has_game_rule = legend_cost_very_low in game/common and game/events, by the top-level object it sits in.
legend_cost_very_low

Source: game/common/game_rules/00_game_rules.txt line 478 (key legend_cost), modifiers from game/common/modifiers, names from the English localisation; Crusader Kings III 1.20.1.2.