If you save loose change every day, it'll add up to a considerable amount in a year.
Literal
Every-day, loose-change [object-を] save [nominalizer-こと] [into-に] become-[conditional-ば], one-year do-[conditional-と] considerable [possessive-の] amount [into-に] become will-probably.
ことになれば uses こと nominalization + になる + ば conditional — 'if it comes to be that you save.' 相当の額 ('considerable amount') combines 相当 ('considerable') with 額 ('sum'). だろう signals probability.