Paint quantities differ if using solid or basecoat paint , solid colours dont need to be laquered so cutting down on paint and cost , where as basecoats needs to be laquered , so doubling up on price .
Personally id always etch prime the car after prepping as then your starting with a firm solid grip substrate , then apply your 2 pack primer , flat down and spray top coat .
More paint applied , more paint there is to flat back and polish up .