Depends what is meant by 'rebuilt' and how much work it needs to have done to it.
You can replace seals, bearings, synchros, keys and springs (a typical reco) and it'll be around $1500 including labour in the shops (for remove/replace/install).
And if you've got worn hub/sliders or broken anything like teeth off a gear and/or shafts, forks, rails, diffs etc then the price can jump up super-quickly from there, especially if sourced locally.
A good second hand EVO III gearbox will still set you back > $1000 if you can find one, but its still a lucky dip on the condition unless its been opened and inspected prior to installation.
Your call should be made once your current gearbox has been opened and inspected. If there is physical damage to internal parts, then could be worth buying another 2nd handy box.
Personally even if the mechanicals turn out to be OK, I would still change everything I listed in the $1500 package above + also pay particular attention to replace the 1st/2nd hub and slider which cops a lot of wear and abuse in these gearboxes.