Enterprise pays the most for ERP. If you can create a better SAP, then you will make millions. I am saying as an SAP user, it is good but really expensive.
Deploying ERP for enterprise customers means writing tons of custom modules & processes specific to the customer. That's why integrating an ERP for the first time or switching vendors is costly and risky. Most successful ERP systems for enterprise customers are basically domain-specific frameworks to create customer-specific solutions.
However, there is still a big market for business management software for SMBs. The market is so big and ever expanding that I doubt it can ever be saturated. Aside from english-speaking market, there are even bigger opportunities for localized solutions.