Salvation would seem to be the most likely motivator for their service. The nobility had the promise of compensation to be granted by either the Byzantine Emperor, who had invited the Franks to fight the Turks, or by setting up an independent Latin Kingdom in Palestine. The Emperor had expected a few thousand merceneries, not vast armies and was inclined to support the crusaders personal ambitions as he could not employ them all.
The peasants who led the way and later accompanied the knights across Europe and Anatolia had little chance of survival, much less material gain in wealth or social position. Medieval society simply did not accommodate such moves, so salvation and everlasting life were offered instead.
The true first crusade was not really a military operation at all. A peasant rabble, led by a mysterious character known as Peter the Hermit, crossed into Anatolia despite warnings from the Byzantine Emperor that Peter's so-called army was no match for the Turks. The Emperor's prediction was fully realized when the the Turks almost totally annihilated the weak invasion force.
The fact that the next crusade, this one composed of a professional army at its core, was able to retain a sufficient number of peasants to serve as the rank and file testifies to their willingness to die in the a holy war. News of such a disaster would have probably reached the army as it arrived in Constantinople shortly after Peter's defeat and although the professional army fared much better against the Turks when they finally crossed the Bosphorus, the prospects for the poor did not improve greatly.
The Tafurs were a particularly zealous group best known for their poverty, determination and ferocity. This peasant army advanced barefoot across the desert, gaurding livestock, foraging for supplies and even turned a few key battles in the knights favor. Unable to afford food when the army was counter-seiged at Antioch the Tafurs it is reported that the Tafurs developed a taste for roasted Turk.
King Tafur then led his men in what must have been a spectacularly well planned flanking maneuver against the opposing Turkish army. The exhausted crusader army had been saved by beggars and baggage handlers who would take no part of the spoils of the richest city in the Middle East. This informal army would perform similarly at Jerusalem again for no earthly reward.