Lakeland can supply you with exact details about electricity use, so you can, if you like, do a calculation.
Leaving aside the environmental issues, I remember reckoning that it would take a good few years to recoup the cost. However, I got one when we were in temporary accommodation with no proper cooker,and I too, take it to use in our camper.
I too use my slow cooker for 'dry cooking' - they really are quite different, and I see little cross over.
The main Remoska advantage is the lower electricity use, and in certain circumstances, the space saving. It does a lot of cooking well, and a few things better than conventional ways of cooking - I love a Spanish omelette done in the Remoska.