What is a lumbar strain?

A lumbar strain is a label on a visit note, and a label is not a price. Before any quote arrives, it helps to know what the label actually claims.
What a strain means
Wikipedia's entry on back strain describes the mechanism. Due to back strain, the tendons and muscles supporting the spine are twisted or pulled.
That is a muscle and tendon problem, a narrower claim than "something is wrong with my spine." Wikipedia also says how it can begin. Acute back strain can occur following a single instance of over stressing of back muscles, as in lifting a heavy object.
Why the label comes before the quote
Treat any figure you are handed as unverified until you know which services it covers. A diagnosis wording that stays vague makes a quote harder to judge.
What to do next
Ask the office to write down the exact diagnosis wording and every service attached to it. Then check the label against two plain references, the OrthoInfo low back pain page and the NIAMS back pain overview. For the service names that show up on a quote, the MedlinePlus pain page and the hospital system's pain management guide are good places to read first.
Confirm the diagnosis before the number
A strain is a diagnosis worth confirming, not a reason to accept a number. In Garfield or anywhere else, get the diagnosis and the itemized services in writing before you compare one office with another.