A very elementary way of projecting the impact of investment fund fees

investment_fund_fees.py

def calc(principal, fee_pct, avg_growth_pct, annual_contrib, years):
    """
    >>> calc(10000, 1.5, 7, 5500, 35)
    (154256.73, 48243.27)
    >>> calc(10000, 1.0, 7, 5500, 35)
    (168507.21, 33992.79)
    >>> calc(10000, 0.5, 7, 5500, 35)
    (184508.4, 17991.6)
    """
    fee = fee_pct / 100
    avg_growth = avg_growth_pct / 100 + 1
    total_fees_paid = 0
    for x in range(years):
        principal *= avg_growth
        principal += annual_contrib
        cost = principal * fee
        principal -= cost
        total_fees_paid += cost
    return round(principal, 2), round(total_fees_paid, 2)

I was comparing Fidelity and Vanguard index funds, and discovered what an impact a small difference in fees can make. In this (rudimentary) example above, the difference of just 1% in annual fees over 35 years can make a $30,000 difference in your portfolio!