01Separate the commodity from the advantage
Accounting, email and payments are commodities — buy them, and do not employ engineers to reinvent them. The part of your process that competitors cannot copy is the part worth building.
Most businesses get this backwards: they accept a rented tool's opinion about their differentiating process and build custom versions of solved problems.
02Count the workaround, not just the licence
The real cost of an ill-fitting SaaS is rarely the invoice. It is the spreadsheet beside it, the two hours a week of re-entry, and the deal shape you cannot support. Price that before comparing to a build.
03Configuration is the middle path
Often the right answer is a bought platform plus a thin custom layer: your rules, your integrations and your interface on top of someone else's infrastructure. It is cheaper than a rebuild and escapes the worst of the constraint.
04Ask the exit question
If you had to leave this vendor in a year, could you? Data export, integration surface and contractual terms decide how expensive that day is. Build the answer into the decision now, not during the migration.