$21/user/month billed annually in packs of 10 (what your source says) would be $252/user/year in $2520 increments. It's actually (per the GHE features page) $2500/year for each 10 user pack though, which is $250/user/year.

GitLab Enterprise Starter is $39/user/year, and GitLab Enterprise Premium is $199/user/year.