。
Going by airplane will be expensive, you know.
Literal
Airplane [by-で] go [if-と] expensive-turns-out [feminine-わ] [assertion-よ].
~と here is the conditional: 'if/when you go by airplane.' 高くつく is a set expression meaning 'to end up being expensive' — つく carries the sense of 'it turns out that way.' The sentence-final わよ combines feminine わ with assertive よ for friendly emphasis.