Building a powerful navy and maintaining an absolute advantage in both quality and quantity over a competitive rival—such as a formidable power or group—is an extremely expensive endeavor.
Just how expensive? For instance, the cost of the "Sovereign of the Sea" was as high as 115,586 Guinean gold coins, with the gilding alone costing 10,691 Guinea. This is the "discounted" price of the ship, constructed with a large infusion of slave labor from the Southern Colonies and without paying a single penny in wages. Lesser-tiers of battleships aren't as exaggerated in cost, but a first-rate battleship excluding the cost of cannons still amounts to 70,000-80,000 Guinean. The battleships of Charlemagne at the same time were similar in terms of price.
