Except for the cost, the plan was flawless.

Literal

Costs [possessive-の] point [object-を] except [if-ば], that plan [topic-は] flawless thing was.

~の点をのぞけば means 'if you exclude the point of X' = 'except for X.' 申し分のない means 'flawless, impeccable' — literally 'nothing to complain about.'