Per person? Since the mid-1700s. As a government? After the Constitutional Convention ad the revolution in financial institutions that occurred thereafter.

A little known fact is that, per capita, the 13 colonies hevne before independence were some of the richest territories in the world. As a Peter Lindert of UC Davis explains:

In 1774 the American colonies had average incomes exceeding those of the Mother Country, even when slave households are included in the aggregate.

High amounts of available land which could be bought relatively cheaply created a system which economic historians termed the "agricultural ladder", wherein a tenant farmer in the colonies was able to save his way into buying his own land - an option not usually available in the finite landholdings of Western Europe. This made acreage per agricultural laborer higher in the United States than in Western Europe, and also meant that, at the time of the Revolutionary War, over two thirds of white adult males were self-employed - most of those as agricultural landowners, the rest as a mix of artisans, merchants, entrepreneurs, and salespeople.

Of course, individual health did not necessarily translate to financial wellbeing for the government, as rates of effective revenue collection in the 13 colonies and the United States under the Articles of Confederation were some of the lowest in the world. The Articles created a loose confederacy that represented the modern day European Union more than they did the present United States. The army units stationed in the Northeast by the time of the constitutional Convention were on the verge of mutiny as a result of the government being behind on pay, and hyperinflation plagued the infamously worthless revolutionary and post-revolutionary currency. During Shay's Rebellion in 1787, the government was notoriously unable to finance an army to put down the mutiny which had seized a US armory.

Several events turned the government's financial situation around. First, the really quite partisan Constitutional Convention - supported mainly by a class of publishers, the upper middle class, businesspeople, and those most victimized by economic insecurity - created a Federal Constitution which granted far greater revenue gathering powers to the central government. In the early United States, there was no consistent Federal income tax, but revenues were raised through import dues, excise taxes, and a variety of "indirect" fees and taxes on the operation of specific businesses or gaining of licenses. The lion's share of revenue were a result of the first - tarriffs, especially as economic conflicts in Europe led to the United States becoming a hub for "re-export" business, whereby merchants intending to trade with a country at war with their own country sold their goods to a neutral port, whose merchants then sold their goods to the enemy of the first country.

Second, reforms under the enterprising early Union financial chief, Alexander Hamilton, restored economic stability to the government. Hamilton created the First Bank of the united States which took on state debts from primarily Northern states and restored financial stability to those troubled governments. This development also allowed the government to borrow to cover temporary expenses, as the existence of a federal debt that was being consistently paid off and serviced made the government a reliable debtor with a good credit history for private banks. More importantly for the finances of the Federal government, he cracked down on tariff evasion by pushing through congress the creation of the "Revenue Cutter" maritime patrol fore, a precursor to the Coast Guard. He further imposed excise taxes, the most famous of which was the Whiskey Tax, which almost led to armed rebellion, quashed by an overwhelming show of force. The imposition of the Whiskey Tax in 1791 was a turning point for Federal Power, as it set the precedent for expansion of Federal indirect taxation. Finally, Hamilton assisted in the negotiation of the Jay Treaty in 1794, ending British economic embargo on the United States and multiplying tariff revenues. What made Hamilton's turn around successful was that, critically, the money was there. As I mentioned, the US had a strong per capita income in the late 1700s, it was simply being under-taxed by the central government due to a lack of real powers granted by the Articles of Confederation.

By 1803, the Federal budget was stable enough, and its credit good enough that it was able to purchase Louisiana with a mix of a $3 million down payment and debt to cover the rest. The Spanish acquisition was less expensive because, unlike the Louisiana Purchase, it was essentially coerced. Andrew Jackson had already crossed into Spanish territory to fight natives, so it was clear to the Spanish government, beleaguered by revolutionary wars in South America, that the United States had every ability to seize Florida by force.

In short, cracking down on tariffs, creating a national bank to service the debt and "build credit", and imposing new taxes overcame the debt situation in the United States and allowed the government to borrow for one-time expenses at reasonable interest rates - such as purchasing Louisiana.

Lindert, Peter and Williamson, Jeffrey. American Incomes 1774-1860.

Irwin, Douglas. Revenue or Reciprocity? Founding Feuds over Early U.S. Trade Policy.

Broody, Kathryn. The Common Thread: Slavery, Cotton and Atlantic Finance from the Louisiana Purchase to Reconstruction.