A rough yardstick is that most enterprise-focused SaaS companies tend to get about 10%-20% of their revenues from professional services.
Now how much to invest is a different question. Many enterprise SaaS companies lose money on services, and only provide them to support the deals and deployments — and success. In that case, implicitly, they are spending more than that 20% to help their customers.
But some do make services profitable, for sure.
Net net, in bigger deals ($50k-$100k+), charge for services if you can. Up to 20% of the ACV is usually OK and you will still be seen as a software business, not a services business.
And then do them as inexpensively as you can — but don’t sweat it if you just break even on services. That’s usually good enough.