Each card gives a base amount of exp according to rarity.

C = 1, UC = 2, R = 5, SR = 20, SSR = 50

Starting at level two, every other level gives 1 extra exp. In other words, the exp given rises by 1 every even level.

lv2 C = 2exp, lv3 C = 2exp, lv4 C = 3exp

If the card it is being combined with is the same element, it gives 1 additional exp.

lv2 same-element C = 3exp

Each card may randomly give bonus exp. This is calculated per card. No card has a higher chance than another of giving bonus exp.
If bonus exp is triggered, then the total exp given is 150% of normal. lv2 same-element C w/ bonus = 4.5exp

Decimals are dropped after adding all the exp from a single combination session (up to 10 cards).

Combining with 10 cards at once is cost-effective, but does not reduce the exp needed to max a card.

Evolutions do not increase the amount of exp a card gives. A C+ will give the same exp as a C.