If the transmission is in good working condition and includes absolutely everything required for a conversion, try to offer him $800 - $1000.
To put it this way, i paid approximately $500 for my M47 conversion for a 740. But mine was missing the pedal box, driveshaft, and master and slave cylinder and clutch kit.
Considering you have all those and it seems from the photos everything is either new or in mint condition, that would be a fair price.