It seems like it was a very dangerous job. Were they promised great riches? Did they get what they were promised? Do we have any accounts of the lives of rank-and-file soldiers after their conquering days?
Conquistadors were not really soldiers in the modern sense. They were not paid a salary, so their recompense was typically in looted goods, preferably gold and silver. However, their motivation to go and become a conquistador was to win an encomienda, which is a quasi-feudal institution where a population of 'indians' were given as a labour force to individual Spaniards. Of course, getting an encomienda was closely related to wining honours from the Spanish Crown, including coats-of-arms and other forms of status recognition. Thus, the ultimate goal of each Conquistador was to achieve social status. When a Conquistador had achieved this, they would settle on an estate, or more frequently, in a 'newly founded' Spanish city in the New World and live as a noble citizen.
At least, this was the 'ideal.' Many conquistadors did not receive much wealth from their conquests, as many native societies were not as rich in gold and silver as many of the Spaniards had assumed. Furthermore, the the rights and labour grants they thought they were entitled did not always materialise, either because the labour was not available, or because the Crown did not want to give too much political and social power to what was essentially a band of raiders. So many conquistadors kept on going, heading to the next raid in the hope that it would prove more productive than the last.