Delegates nominate. Primaries and caucuses exist to secure delegates for the nomination.
According to The U.S. Presidential Nominating Process by the Council on Foreign Relations:
In 2016, a Democratic candidate must secure at least 2,382 out of 4,763 delegates to become the party’s nominee. The number of delegates allocated to each state takes into account the state’s Democratic vote in the previous three presidential elections and its assigned number of Electoral College votes.
Meanwhile, a Republican candidate must secure at least 1,237 out of 2,472 delegates to win the party’s nomination. The Republican Party allocates each state ten delegates, plus three for each congressional district, and bonus delegates for states that contributed electoral votes to the party in the previous presidential election, as well those that elected Republicans to high offices.
Right now, at his pace, Jeb Bush would need to spend $21.03 billion to secure the nomination. Marco Rubio would need to spend $4.771 billion to capture the GOPher party nomination. John Kasich would need to spend $4.36 billion.
Jeb, Rubio, Kasich — they are not viable considering the national election might need another billion or two in spending.
Meanwhile, self-funded billionaire businessman Donald Trump would only need to spend $509 million to win the Republican nomination from restorationist voters.
Jeb Bush was the Republican Party establishment's anointed candidate. Bush would need to spend 97% more than Trump. Said another way, for every one dollar Jeb Bush would need to spend to capture the nomination, Donald Trump would need to spend only two pennies.
It's the price per delegate and securing the most delegates that counts.