how much labor cost to replace the pull cord spring on a 128ld husqvarna weed eater?

Just replace it yourself, it isn't hard to do.

Download the parts manual, which should show you how things go together.  Remove the pull start housing and carefully remove the damaged cord - I say carefully because the rewind spring is in there too, and lots of people have no experience getting that thing back in there.  I've done it enough times that replacing a cord or even the spring is nothing to me anymore, just a bit on the messy side.

Getting the old spring out is pretty easy, get the cord pulley off and often the spring comes too - a bit violently.  Be warned.  If that isn't a problem then you can carefully extract the spring one coil at a time, retaining the rest with a thumb as you let the tension off what is extracted.

If the new spring has retaining wires on it, leave them on until you've tucked it into its new home, and then carefully clip the retainers and wiggle them out of there while holding the spring in place.  Then you can lay the pulley back in and get the cord back on.

What you do to add tension to the cord is this: either wind enough cord on that you can then pull it out and install the handle at that point, leaving sufficient tension in the starter, or else wind on what you have, pull the cord out until you can pin the spool in place (some have a pinning hole right in them for exactly this reason, just poke a screwdriver or something through them to hold them still) and then wind the cord you pulled out, back onto the spool by wrapping it around the spool while the spool stays still.  Then install the handle and boom, done.

It sounds like a lot and the first couple you do kinda are, but after that it's an easy job and you won't shy away from it at all.

I just replace one - part was 14 bucks 3 screws (hex heads or torq) on and off in 10 minutes. 

Look up what the price of the repair piece is, add 33% to it, then triple it.

Minimum of $50, but likely more, if it's just the spring replaced.

Might be cheaper to find a used recoil mechanism , either from a small engine/lawn mower shop, or places like eBay.

this is a local handyman or small engine repairman job ... bargain in advance on the cost.