Reach RS+ Resale Value UK

I’ve just ordered two RS2 units to replace my two RS+ receivers which are now surplus to requirements. They are less than a year old, just wondering what a fair resale price would be? The local college are looking to purchase a pair for introducing students to GNSS surveying so I want to give them a fair price (considering they are eligible for 20% discount on new ones).

Thanks,

Rory