After a few uses it seemed like it doesn't give much. I''ve decided to add a few more tries, and for 300 WE I've received the following bonus materials:
2x sword
3x leather armour
2x wooden shield
1x rugged clothes
1x warm cape (the only 3*)
Figure out how long it takes you to spend 300 WE for 10 additional ascension materials.
Compared with none-bonus ascension materials, which I got 1 item per 1 WE on average, it's a 3% increase. And not a single added backpack, mind you.
My verdict? Not worth it