(pp. 25) Default starting wealth (Average Wealth) is $1000. Characters may choose to be wealthier, or poorer, but it's worth bearing in mind that this is an apocalyptic survival campaign and the characters won't have many opportunities to use their wealth after character creation (although you never know as the campaign progresses). Characters who choose an above average wealth should invest it in things which will help them survive the initial collapse of society, and rebuild it after the disaster ends. So all levels of the Wealth trait are allowed but players should be pragmatic about it. If someone wants to put all their points in to a rich merchant who is suddenly broke because the systems of credit and banking broke down then that would be mostly for RP-flavor, and the GM will probably give a point discount unless the wealth can be used at some point later on (an interesting background idea). The starting region ( The Kingdom of Nool) is a medieval society with some concepts of banking and credit, where most deals are done with coins or in-kind-transactions. A wealthy survivor will have to decide how and where their wealth is stored and what it represents before the game starts. Those details might influence the cost of the trait, depending on how useful the wealth is in the campaign. Is it land? Property? Valuable items? Gold in a vault? Gold in a bank? Gold in many banks? Something else? Many possibilities.
The Crisis doesn't care about your wealth, but survivors might. The Kingdom of Nool isn't the only economy in the game universe (Outlanders might even have come across paper money if they come from far away enough). Pockets of 21st-century-equivalent society do exist (but nowhere even close to Nool) and it's not impossible for a character to literally have money in a digital bank somewhere. Since wealth can mean a lot of things here, the point cost will mostly be determined by how useful it is in the immediate starting circumstances. This means an Outlander might have been the wealthiest person from his home region, but have no way of converting that money to utility in the Kingdom of Nool, which could mean several different things regarding the way GURPS handles wealth. This will be handled on a case by case basis.