My first instinct as a software dev is that coding up a prototype of a billing system can be completed more quickly than evaluating two or three vendors of billing systems.
Either way you can be highly successful or deeply disappointed. What really matters is the match between the mental model behind the system and your domain.
In principle the product you buy can have a lot more labor in it both in terms of design and implementation. On one level you benefit from outside investment into vendor but on another level you (and the investors) are paying for sales, marketing and management at the vendor too. The commercial product almost certainly has a better looking UI for your team to use. Something you develop yourself will likely have a minimal UI but if it meets your needs than that's good enough.