Buying software off the shelf or custom building will cost the same amount of money in the long run.
Time and time again people go for the cheaper solution in the short term and end up paying just as much if not more for the privilege as the years go by.
Break fixes. More dev heads. Integration. Money. Improvements. Planning meetings. New Products/Services. More money. More time. More energy.
Do you want to hear the only thing that is different? SPEED
Your speed to market.
Your speed to serve your customer.
Your speed to add a new product.
Your speed to create new relationships.
Your speed to communicate.
Your speed to hire.
Your speed to reach break even.
Make a decision to custom develop your software capability on whether or not it is your IP not on how much it will cost you in the first year.
If you make the decision on first-year costs alone you are headed to spend just as much money in the long run but sacrifice your speed.