You need to think and look like this ... cost increases exponentially due to 2 factors:

1. Tier items (lvl 5, lvl 15, lvl 55) drop rate lowers with higher tiers (meanong even at lvl 55 you will get muuuuuch much more lvl 1 than lvl 55)
2. Tier stats on items are even lower.

Lets say % drops are: 25%, 15%, 12,5%, 10%, 5% all the way to 0,001% for tier XIV (lvl 55) for point 1. Then you have same % (but actually really lower for point 2).

Chance to get tier 55 dager 270 ee is like 0,001% x 0,000001%