I do a lot of these boxes and to be honest you dont know what the costs are before its in pieces. Sometimes the 5th gear wont come off without grinding; then you will need a new one which of course mean higher the costs. Very often the dogs on the gears and selectors (hubs) are worn out due to rapid gearing / abusive cossie owners, hence these will need to be replaced.

Any workshop who do these transmissions will (hopefully) tell you this; if they dont and only fit a cheap 200p overhaul kit + labour costs then most likely the box will fail very soon. It wont help just fitting new syncrorings when there are 3 parts to the syncro mechanism. (gear, syncro hub and syncro ring)