If leaking from the lineset - the tubing running between the components, that can be cut and spliced - or just a section of lineset replaced for probably somewhat less than that $600 labor charge (though still likely $300-400 or more).
If the leak is in a line in the evaporator itself - part of the "coil" - just solder is not an adequate fix for that. Solder is a joining material, not a patch or filler. In the old days it would have been brazed (basically gas welding), if at an accessible point - but these days due to liability and labor costs very few contractors do that sort of repair on a residential unit. (They are still done on large commercial units).
As for the warranty - read your warranty - they vary by manufacturer. "Last time" might have been with a manufacturer who covered parts and labor (few do today - most don't even have enough consfidence in their product to cover replacement coil shipping to you and return of bad one to them). Or might have been in first year of warranty so covered labor, but for remainder of 5 or 10 year or lifetime warranty (whatever you have) only covers the defective part, not labor. Also - if in the lineset, generally that would not be covered past the initial warranty period - 3-12 months typically.
And check manufacturer AND installer warramnties - might be last time you were covered under say a 1 year installer warranty that did include labor, with the parts being covered by the manufacturer.
IF you check out previous similar A/C evaporator coil replacement questions in the Homne > HVAC link in Browse Projects (at lower left) you will find $600 is in the normal range that has been quoted or ballparked for coil replacement labor and incidental materials - not including the coil itself.
