A noble in DnD would have between 10,000 and 1,000,000 gp in liquid assets, depending on how high they are in the nobility, the general wealth of the region, and the Game Master’s discretion for the state of the local economy.

We can look at a few rules guidelines from 5e’s source material to land at this number:

• Lifestyle expenses. A wealthy lifestyle costs 4 gp/day, while an aristocratic one costs a minimum of 10 gp/day. This works out 1,440 to 3,650+ gp/year in expenses for a single person with this lifestyle. Take into account that a noble house would have several members, and annual expenses easily exceed 10,000 gp/year for most noble lords.

  Taking a corollary from the real world, most wealthy people have enough money in savings to cover at least one year’s living expenses for their household. Of course, real-world people and fictional nobles alike are prone to “lifestyle inflation,” where they’re completely dependent on credit to maintain their lavish lifestyles. Meaning not all nobles will have a large sum of liquid wealth.

  Still, it’s safe to assume that most nobles would have at least enough gold on hand to cover one month’s expenses (120-300+ gp multiplied by the size of their household).

• Maintenance costs. A palace or large castle requires 400 gp/day to maintain, working out to 146,000 gp/year. This takes into account wages for 300 hirelings. A noble estate costs 10 gp/day to maintain, or 3,600 gp/year. So a noble with a palace or castle could be expected to have at least a percentage of their annual expenses on hand at any given time.

• Labor costs. A skilled hireling costs 2 gp/day (~520 gp/year, assuming modern weekend standards). This would be the salary of the equivalent of a middle-class earner in today’s world. If we assume a “rich” person has between 10 and 100 times the wealth of a middle-class person’s annual salary, a noble in DnD would have between 5,200 and 52,000 gp.

• Costs of relevant items. A suit of 15th-century plate armor would cost anywhere from $8,000 to $40,000 by today’s standards. Let’s take the average of that and assume plate armor costs $24,000, while in DnD 5e, plate armor costs 1,500 gp.

  Doing some back-of-napkin math, this means that 1 gp = $16. If a “rich” person today has $1,000,000 net worth, then a rich person in DnD would have a net worth of 62,500 gp. Note that “net worth” isn’t just liquid assets; it includes their property, investments, etc.

  Another way to look at it: if a noble has a standing army of 50 soldiers equipped in average-quality plate armor, they spent 75,000 gp ($1.2M) to do it.

Hopefully, these guidelines from the PHB and DMG, along with some (admittedly flawed) estimates help you decide how much gold nobles have in YOUR campaign.

Player’s Handbook, pg. 157
Dungeon Master’s Guide, pg. 127