Diocletian, all other faults aside (sticking Christians like marshmallows, knocking down churches & burning Alexandria were all favored policies), was likely the first Roman emperor to fully grasp the peril of his people and realize Rome’s apogee had passed. In the late third century under his reign, the standard of living in Rome slipped badly for many people, and Diocletian was shouldered with the much of the blame. However, inflation and scarcity were economic conditions borne of the policies of his precursors. He’d simply inherited command of an Order on which a long-darkening shadow had finally fallen.
      The stage was set at the end of the Germanic Wars in 16 AD, which marked the apex of Roman expansion, and the increasingly necessity for pacification – Rome’s frontiers now extended across the Balkans to the Black Sea in the Near East, and out to the tip of Gibraltar in the West, and to the Rhine in the North. Yet ever and on the people kept coming – Picts and Celts, Saxons and Angles, Getans and Lombards. Each new wave of immigrants had to be dealt with as peacefully & quickly as possible in order to maintain order. In 231, Gallienus had ceded vast swaths of farmland in Pannonia to the tribal King Attalus, in return for peace on the borders and men to man the great ’limes’, fortress walls, under construction throughout Germania and the northern realms of the Empire. Thirty years later, in 268, Claudius II did much the same with the first wave of Visigoth migrants who poured over the Rhine, settling them within the outer provinces as ‘coloni’. When Diocletian inherited the imperial purple in 285, he followed the precedent set by the policies which came before him, continuing to settle barbarian immigrants (Suevi, Goths, Franks, etc.) wherever space might be found for them, so long as they swore allegiance, cleared the land and some took military orders.
      As mentioned above by monkeylover, “Diocletian attempted to reform the struggling Roman economy, but realized that there wasn't enough hard currency to regain confidence or strength in it.” All the hard currency was in essence flowing out of the major Roman cities to fund the soldiers, carpenters and farmers of the border regions – and as much of the buying power was shifted to the provincial communities and frontier towns, prices in the major urban centers like Rome began to rise sharply. By the late 3rd c., some estimate monthly inflation may have reached 10% for some staples. Suddenly, the comfortable middle class (what little there was) was no longer so secure, and with that uncertainty came immediate unrest. The same bureaucrats and merchants who had demanded protection from barbarian incursion now demanded protection from the costs of that protection – and domestic affairs worsened. Diocletian issued his Edict on Maximum Prices in 301 AD – after century of Roman placation towards its provincials and barbarian neighbors, the first true state attempt at wage freezing and price control – after of course finding a suitable scapegoat for the problem (which was always crucial in Roman oratory politics). Here’s how it began :
Is anyone so dull and unfeeling as not to know, not to have seen that the high prices in our markets, on which the daily life of our cities depend, seem not to be checked by either abundance or bumper crops? So abandoned is the Passion for Gain that men in the businesses actually try to control wind and weather by the movement of the stars and wickedly abhor rains which make fields fertile, for the abundance which favorable weather brings they calculate as a loss…men with enough wealth to satisfy whole nations try to capture smaller fortunes and strive after ruinous percentages. Concern for humanity in general persuades us to limit the avarice of such men.”
      So Roman authorities and soldiers in the markets imposed price caps on many goods, and the free market of Roman merchants and middle-men was reigned in. However, scarcity and high taxes in the cities remained a visceral reality, greatly limiting the spending power of the vast majority of the population. This expansion of the military under Diocletian, now well over half a million men under arms, was simply too much for the tax base and fragile economy to support. Areas with more fertile and better irrigated soil or more-developed trade (i.e. the central regions of Italy, Greece, Egypt, etc.) were taxed ever harder than the newer, frontier districts. But the Edict of Maximum Prices, which was to fix prices and wages throughout the Empire, only complicated the economic crisis - trade ground to a standstill as the impetus to move goods collapsed. Many items became so suddenly unprofitable, they simply vanished from the marketplace. By 305, despite all his attempts to restore the integrity of coinage and reign in imperial budgets, Diocletian was forced to abdicate to Constantine – who believed social policy in the Eastern Empire, not the economics of the West, would save Rome.
Sources: Edictum Diocletianide Pretiss Venalibus, ed. Blumner; H. Hada History of Rome: from its origins… (NY: Doubleday, 1956), 179; “DIOCLETIAN” - http://www.romanempire.net/decline/diocletian.html; “Diocletian (284-305 CE) and Constantine (308-337 CE): Efforts to Stabilize the Economy” - http://www.fordham.edu/halsall.

Images: Antoninianus coin minted under Diocletian - http://www.usask.ca/antiquities/coins/diocletian.html