The best kind of "problem" to have. Expect to build features, integrations and other customizations, and factor in the costs. You can do that with a higher "base" price, or include professional services into the contract (but be careful with IP rights). Use a per-seat model internally (more users = more support etc) and offer fixed price point per year. Expect to them to want to negotiate a large discount on the "list" price, so start higher than you might think.