Hello Simon,
Hard to say what is the value as there are many factors :
Consider a basic UN1-13 to be an average of 300 euros of value (from junkyards or others).
Consider the value (renault prices) of parts to be replaced (serious rebuild) of an average of 300 to 600 euros (would depend if you chnage synchro rings or not).
Consider the time : dismantling, cleaning, checking parts, blueprinting, assembly, setting the right torque on diff (wich means opening, closing, reopening, closing etc..until you reached the correct value) so basically 2 days of work.
Give a value to that time and you will have the value of the gearbox.
Cheers
Stephane