Speaking only in terms of the 10th-11th centuries, which is the time period with which I'm most familiar, you didn't buy your way into nobility--you postured your way into it.
People at the time just didn't separate a person's appearance of status from their actual status. The difference between a nobleman and a rich peasant was all pageantry. To be a noble, you had to visibly represent your splendor and have it recognized by other nobles. A noble had to demonstrate that he was a noble by having the material assets of a noble, having the dignity of a noble (etiquette and recognition), and patronizing his entourage and the local church with gifts. A noble who didn't do noble things risked having his rank fall back to that of a rich peasant. A rich peasant was someone devoted more to agriculture and military service when required.
A noble household had to have a certain number of servants and retainers. A lord had to throw generous banquets and pass out gifts to his entourage, like clothing, horses, and swords. A lord had a sword, good horses, and clean and honorable clothing. A king had elegant vassals.
Otto the Great would process back and forth to and from the church on feast days in the company of clergy carrying candles (wax candles were relatively rare and expensive at the time), censors, and crosses. It displayed his prestige. Other aristocrats showed up just to watch a king show how kingly he was, especially if he was meeting with a rival king.
A noble's honor had to be recognized in order to exist, and a higher aristocrat's failure to recognize a noble's honor questioned whether they were actually a noble. Minor deviations from standards of etiquette could result in huge problems. In Casus s Galli,Ekkehard of Saint Gaul recounted how one bishop greeted the king's chamberlains, who refused to greet him back. Eventually, the king had to order them all to make peace, because by failing to greet the bishop back, the chamberlains had attacked the bishop's social prestige. And when Louis III said that a Tuscan Margrave had such a great entourage that he should be considered a king, it created conflict in Italy.
This question depends entirely upon one’s definition of commoner and noble as it related to the Medieval Holy Roman Empire. The term “prince” or “fürst” in German, referred to a chief man, it could “mean the king himself, the free-born nobility of the Empire, the prelates of the Church, and even the ministeriales of the crown.” (see source below) Now it’s important to define what a ministerial is.
Nobles were born free and exercised the right of self-determination. By enlarge, mostly everyone else in that society was born into a manorial, feudal system in which the peasants were sworn to their master (the Swiss Kantons being a notable exception). They were un-free and could not, for instance, bear arms without the permission of their lords. With the expansion of powerful families in the 13th century such as the Habsburgs, most of these families relied on members of the unfree peasantry to act as ministers, castellans, knights, and other important roles that elevated them above other unfree peasants. In fact, most of the armies of knights in Medieval German battles consisted of ministerials. They were still sworn to their lord, but the gap between ministerial and noble becomes increasingly blurry in the fourteenth century. According to historian Benjamin Arnold, “functions turned out to count for more than did free birth. Ministeriales, like knights elsewhere in Europe, were becoming acceptable as noblemen because they fought as cavalry, attended the crusades, lived in castles, towers or other fortified dwellings of aristocratic appearance, exercised seignorial and other jurisdictions, and were generous patrons of the Church, amongst their many honorable functions.” There were also rare circumstances of intermarriage between nobles and ministerials. According to Arnold, “The limits were not often transgressed, although there were already cases in the twelfth century where daughters of counts might wed prominent ministeriales, and in the thirteenth century, free-born lords married the heiresses of very rich ministeriales.” But Arnold clarifies that “the high aristocracy of Germany was successful in preserving its identity as an endogamous caste until the fourteenth century.” Prior to that, marriages between nobiles and ministerials were rare, and children of such unions were required to procure certificates of emancipation, lest they should lose their personal freedom.
In summary, over generations, an un-free peasant, could make their way up the social ladder to a place of distinction that somewhat resembled nobility.
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