Depending on route travelled
Normal SVR price for this journey after 0900 M-F / anytime weekends/public holidays is £53.60 before any railcard discounts are applied.
Going direct, follow the advice above
Going via Leicester, best split would be at Leicester itself
Ely - Leicester: CDR £19.50 (not valid on 1619, 1719, or 1807 - though I doubt you'll be travelling that late)
Leicester - Chesterfield: SVR £16.30
Total £35.80 - saving £17.80
There will probably be a cheaper fare if split again between Leicester and Chesterfield