You don't want to get into specifics
Ultimately, the calculation of gross national product (GNP) is a pretty in-depth measure. Not only do you have to calculate the value of all goods produced, you also would have to calculate the value of all services provided. That means you have to know all the good produced and services provided on your world.
This is harder than it looks. The Bureau of Economic Analysis in the United States identifies 403 separate industries that it analyzes. Do you know what the contribution of copper, nickel, lead, and zinc mining is in our world? Well, make sure you don't confuse it with iron, gold, silver, and other metal ore mining since that is a separate industry. What about the manufacturing of abrasive products, irradiation apparatus manufacturing, carpet and rug mills, news syndicate, lessors of nonfinancial intangible assets, services to building and dwellings, home health care services and religious organizations?
The point is, you don't want to be stuck quantifying all these individual fields to attempt to come to some numerical solution.
I would suggest simply doing research on a current day or past nation, and carry forward the same ratios of expenditure.
For example, if you use the modern day US, a filthy rich player character (PC) might be worth a billion dollars. A brand new aircraft carrier costs 13 billion dollars; not affordable. Meanwhile, an M1 Abrams main battle tank cost the Saudis about $10 million each. So that would be affordable.
Going back in time for a different recommendation, a study of England in 1900 turned up 540 business estates worth an average of 590,000 pounds. So you might estimate a filthy rich PC would be worth that much. Meanwhile, HMS Dreadnought cost 1,785,683 pounds to build; relatively cheaper than an aircraft carrier is today.
If your planetary society is more like the US (more democratic, higher floor for standard of living, low birth/growth rate) or 1900 England (more aristocratic, lower classes in relative squalor, high birth/growth rate), you can choose a set of numbers that suits you. The same method can be applied to basically any other goods or services that you want your PCs to purchase.