To be somewhat expensive. This intentionally vague phrase highlights the speaker's awareness of or hesitation about the cost of something. That car service is a bit on the expensive side. Can't we just take a taxi?

To partially have some quality, typically a negative one. An adjective precedes "side." The dress is beautiful, but it's just a little on the expensive side for me. A: "Do you like what you ordered?" B: "It's good, just a little on the salty side."