Summary:
After uses charge (purge) of Diffusal Blade the cost isn't changing. The same with Diffusal Blade level 2. In Dota 1 the cost changes.

Repro:
1. Buy Diffusal Blade.
2. Use charge.

Result:
New cost isn't changing.

Expected:
After uses charge the new cost must be equal to (full cost)/2*(charges left)/8. With 0 charges the cost must be 0.
The same with Diffusal Blade level 2.