It's a bit complicated.

Usually, while we calculate the offer for the buyback, we do this following way:

1. estimate the actual market value of the truck basing upon the brand, year, working hours and applictaion/general condition,
2. estimate the cost of refurbishment (if planned),
3. add the logistic costs (transport etc)
4. add the sales margin.

The difference between point 1 and the rest gives the buyback price.
Of course there's the issue of margin level etc, which varies between 15 and 40%, depending upon the company. Here's the fileld for negotiations.

Another issue are the batteries, which after 7 years are probably dead/empty, so you must be aware, that nobody will pay for them.

Every serious manufacturer has a huge tables showing the residual values of the trucks after LTR. They are based upon the year/working hours/application factors and all are constructed the way I described above.

For example - high class European IC 2-tonner after 72 months/2000 hours a year/medium application has the residual value between 10 and 15% of the purchase price.
High class European reach truck after 72 months/1000 hour a year/medium application is worth for 15 to 20% of the purchase price.
Etc.