Simulate pricing of IBKR

# Why?
- Because it has an impact on investment yields.
- Because I happen to use IBKR.

# Condition of satisfaction
- Implement both FIXED and TIERED price structures.
- https://merelycurious.me/post/interactive-brokers-fixed-vs-tiered-commissions
- https://interactivebrokers.com/en/index.php?f=1590