First, 18 splines bring much more $$$ but to answer your question: it depends.
You have a 23 spline which is much more abundant and most folks will want you to pull the cover off to see how the gears and synchros are.
Fully rebuilt 23 splines from Brewers is around 1500 bucks, used units can range from recently rebuilt 800-1000 bucks down to 250-500 for ones that have had some use and needs some attention.
Hope that helps,
B