Router? Gateway? Firewall? Network Access Control? VLANs? Ability to manage all this through declarative version controlled code w/ rollback?
The costs of doing those (well) yourself are not cheap.
Getting them from a provider that's certified to do them well while giving you software control also isn't cheap.
You're comparing cost of gas per gallon to to expense of miles driven per gallon. Pretty sure on your IRS or corporate expense report those aren't the same.