Coste vs. costo vs. gasto

The Spanish words 'coste', 'costo', and 'gasto' all relate to expenses, but they have distinct uses and nuances.

Coste

'Coste' is commonly used in Spain to refer to the cost or price of something, typically focusing on production or acquisition costs.
El coste de producción es muy alto.
(The production cost is very high.)
El coste del viaje incluye el vuelo y el alojamiento.
(The cost of the trip includes the flight and lodging.)

Costo

'Costo' is more frequently used in Latin America and serves the same purpose as 'coste' in referring to costs or prices.
El costo de vida en la ciudad ha aumentado.
(The cost of living in the city has increased.)
El costo del equipo nuevo será cubierto por la empresa.
(The cost of the new equipment will be covered by the company.)

Gasto

'Gasto' translates to 'expense' and refers to money that has been spent or is going to be spent on goods or services. It can be both fixed and variable expenses.
Hemos tenido muchos gastos este mes.
(We have had many expenses this month.)
El gasto en electricidad fue menor este año.
(The expenditure on electricity was lower this year.)

Summary

In summary, while 'coste' and 'costo' are synonymous with subtle regional preferences—Spain favoring 'coste' and Latin America favoring 'costo', both meaning 'cost or price'—'gasto' specifically refers to 'expense'.