As someone who built a niche SaaS I can see both sides of this.

If you are looking to replicate the exact same feature set of an existing product - buying would almost always be a better choice.

But it's rare for avg customer needs to perfectly match product features. Most often they need 20-40% of the product + some custom, business specific logic that's missing and is later fixed with spreadsheets/integrations. In that case it depends on how mission critical this software is.

I'd say investing into the core software that runs your business might very well be worth the effort, even if it's 10x between build vs buy.