Unlocking a hero costs 10 mementos, and taking its four skills from level 1 to 5 costs 680 more. That is 16 upgrades, and each one is priced by how many the hero already has in total, not by which skill you raise: the 7th costs the same on any of them.
| Upgrade | Mementos | Running total |
|---|---|---|
| 1 | 10 | 10 |
| 2 | 10 | 20 |
| 3 | 15 | 35 |
| 4 | 15 | 50 |
| 5 | 25 | 75 |
| 6 | 30 | 105 |
| 7 | 35 | 140 |
| 8 | 40 | 180 |
| 9 | 40 | 220 |
| 10 | 45 | 265 |
| 11 | 50 | 315 |
| 12 | 55 | 370 |
| 13 | 70 | 440 |
| 14 | 75 | 515 |
| 15 | 80 | 595 |
| 16 | 85 | 680 |
| All 16 upgrades | 680 |