For Alexander, what were the incentives of expanding to the west vs to the east?
If Alexander would have decided to go westwards, would it be as successful and rewarding in comparison to his eastwards expansion.
The east had much more money than the west. To the east lay large empires with huge cities teaming with people and gold. All the major trade routes led to these places. Egypt itself produced vast amounts of grain. Italy's cities were much smaller and their economies relatively tiny. It would have cost far more to invade Italy than any amount that could be recovered there. Douglas MacArthur famously said "Wars are caused by undefended wealth" and that is certainly the case here.
Livy nationalistically claimed that had Alexander invaded Italy, Rome, a tiny city state at the time, would have defeated him, a popular (with Romans), but not very realistic assessment. Strabo wrote that Alexander sent an embassy to Rome to complain about pirates operating out of Tuscany.
It should also be noted that Macedonia was a relatively poor kingdom and the wealth of Persia was on a much higher level, especially for its aristocracy. The Greeks had been aware of this for at least two hundred years. The Persians were also hated by most Greeks. As it has been mentioned, it's easy to motivate people to go conquer a hated enemy AND get rich doing it. Conquering a relatively poor and ineffectual territory? Not so much.