Ignoring the Classical Approach, the above systems have many things in common. The main unit of exchange, whether Industrial Points, Money, or Economic Points, is static in worth from purely a cost standpoint. For instance, if an army costs 5 EP, it costs 5 EP. Whether or not your country has 2 EP, 10 EP, or a 100 EP. The relative cost to you may change, but the hard cost of the army remains the same. People may pay less for an army that costs 5 EP from another player, but they won't pay more.
Money spent leaves the game. When 2 EP is spent to increase EP by 1, 1 EP is leaving the game. When an army that costs 5 EP is destroyed, 5 EP is leaving the game. When EP is put into technology, it usually isn't liquid, effectively "removing" it from play.
Money still accumulates however. Armies, navies, and air wings usually can be refunded, and as the economy grows, the more of those you have, as well as actual EPs.
Another important aspect is that a currency is universal. 1 EP is 1 EP, whether you're Black Hole or CRASH. While some games before have adopted limited currency use, the implementation has been limited to mainly RP and events and haven't really be tied directly into the economic system.
Trade is also heavily abstracted. Trade is largely just a number, brought upon with a formula. MP3 adds three tradeable resources. Consumer Goods, Raw Materials, and Energy. However, for the most part, a player can somewhat ignore these. Somewhat.
Consumer Goods are removed from play by being converted to cash. This cash is put into a domestic bank and the country's vault via taxes. Raw Materials are drained via industry and building new units. Energy is drained via maintenance, industry, and population growth. The MAIN unit of exchange in MP3 is still a global currency (with different names, but still universal and global), and the cost of RMs and Energy can be in flux depending on supply and demand.
Consumer Goods, however, can not. Each consumer good is worth $10. Since 1 IP produces 5 Consumer Goods, that is $50. Therefore, if the cost of RMs and Energy to power 1 IP ever exceeds $50, the economy of the game will crash. However, since the only limit to resource production IS provinces, the flipside of the scenario is possible. Resources are so plentiful that a RGO-based economy simply can not work.
In other words, the most reliable source of income is trade revenue and population.