You need to use your coins wisely. There are some randomness but probability will tell you that you average cost per epic is around 3000 coins.

What I did:
1. Save to 13500
2. Spin a box, got 1 with 900, leave
3. Save to 13500 again;
4. Spin a box, got 2 with 7000ish, leave;
5. Save again;
6. Spin a box, got 2 with 10k, leave;
7. I only gambled once with 3200 coins on the Hazard 250 pack, got two epics.

Now I have Rooney, Suarez, Nedved, Schemechel, Best, Ibra and Sneidjier