There was no problem performance-wise, but the cost per unit was just too enormous, so the cost-performance was poor.
Literal
Performance-wise [topic-は] problem not-was [explanatory-んだけど], one-unit per [to-に] cost [subject-が] too-much enormous [and-で], cost-performance [subject-が] bad [quotative-ということ][confirmation-でね].
性能的には 'performance-wise' — ~的 makes a noun into an adjectival modifier. 一体あたり 'per unit' — あたり means 'per.' コストパフォーマンス is a loanword compound. ということでね wraps up the explanation with a trailing confirmation. A technical/business assessment.