The exchange... What does it give out?

Is it worth dumping in low level cards for a ? Or am I better off using them to enhance?

Here are the odds of getting specific items from her:

http://www.fantasicaunofficial.com/t5560-leana-shop-statistics-also-tutorial-for-shop-5024-bags-now-statistically-meaningful-not-updating-any-more-thanks-for-the-data

She also gives small amounts of some event items, depending on the event. Once you have a lot of excess cards you can start dumping them in Leana.

if you have cards that need enchanting and doing so will greatly help you in events or you know you can sell that card for a profit then enchanting is the way to go.
some people, including myself, save up those exchange points (lp) because in some events you can get very helpful items that will be a huge help in ranking.
all in all if your wanting that 5* edwin, your probably better off maxing and selling your 4* cards for te and just out right buying him rather than going with the shop.
this is what i do: 4/5* monsters & units (not all just the baddies), 3* units go to Leanas. 1/2* unit and monster & 3* monsters are all used as feeders to enchant.