Why so expensive — can I DIY the upgrade?

Yes, if your team has shipped a Magento upgrade in the last 12 months. No, if they haven’t.

The honest cost equation:

• DIY in-house with experienced team: ~60% of the calculator number in cash, but 1.5–2x in calendar time. Worth it if you have a senior Magento dev on payroll.
• DIY with a dev who’s never done a Magento upgrade: You’ll spend 2–3x the calculator number when you count the senior-dev hours wasted re-googling the same composer errors. I’ve cleaned up 40+ “cheap DIY upgrades” that ended up costing more to fix than to do properly.
• Hire a consultant for the audit only: Smartest middle path. Pay $1.5k–$3k for a 1–3 day audit + written upgrade plan, hand it to your team, save the build cost.

Specific risks of DIY: missing a deprecated API surfaces in production a month later (when an obscure cron runs); an untested extension breaks checkout silently for 2–3% of customers; a forgotten db_schema.xml migration leaves stale columns indefinitely. These are the kinds of bugs that cost $20k+ to chase later.