For Apple to make $99 of your sales (not accounting for taxes etc), you'd need to generate a volume of sales of which 30% would amount to $99, i.e. $99 / 0.3 = $330.

If you're asking "how much does a developer need to make in sales every year to recoup their membership fee" that's different: $99 / 0.7 ≈ $142 (rounded up). But that's kinda orthogonal to Apple's interests.