Refactoring for https://www.reddit.com/r/learnprogramming/comments/5wol15/cant_get_my_code_to_do_math_properly/

profit.py

# - Global Constants -
SALES_TAX = 35  # Tax paid when selling (in %)
CURRENCY_UNIT = "USD"


# - Input Handling Functions -
def input_total_cost():
    """Asks the user how much he spent for the products in total"""

    total_cost_raw = input("How much did everything cost? ")
    total_cost = total_cost_raw.replace(',', '')  # Not quite sure what this is supposed to do
    return int(total_cost)  # Why do you want Integers? What about the cents?


def input_number_of_items():
    """Asks the user how many items he bought"""

    number_of_items_raw = input("How many items do you have? ")
    number_of_items = number_of_items_raw.replace(',', '')  # Not quite sure what this is supposed to do
    return int(number_of_items)


def input_item_sell_price():
    """Asks the user how much he can sell one item for"""

    item_sell_price_raw = input("How much does one item sell for? ")
    item_sell_price = item_sell_price_raw.replace(',', '')  # Not quite sure what this is supposed to do
    return int(item_sell_price)


def is_input_valid(total_cost, number_of_items, item_sell_price):
    """Checks if the user inputs are valid"""
    # TODO: Split each check into new smaller functions if they get too big. Now it's just one if, so it's ok.

    input_valid = True

    # Check total cost
    # Nothing to check

    # Check number of items
    if number_of_items < 1:
        print("You have to buy at least one item!")
        input_valid = False

    # Check item sell price
    if item_sell_price <= 0:
        print("You can't sell your item for a negetive price!")
        input_valid = False

    # True if all checks passed
    return input_valid


# - Business Logic Functions -
def calculate_total_sell_price(number_of_items, item_price):
    """Calculates how much you could sell all your items for"""

    return number_of_items * item_price


def calculate_total_sell_price_without_tax(number_of_items, item_price):
    """Calculates how much money you have after you subtract the tax"""

    price_with_tax = calculate_total_sell_price(number_of_items, item_price)
    remaining_ratio = (1.0 - SALES_TAX/100.0)
    price_without_tax = remaining_ratio * price_with_tax
    return price_without_tax


def evaluate_profit(cost, revenue):
    """Evaluates if your investment would be profitable, and prints an according text"""

    profit = revenue - cost
    if not profit > 0:
        loss = -profit
        print("It's not worth it to make this investment. I would lose {0:,.2f} {1}".format(loss, CURRENCY_UNIT))
    else:
        print("I would make this investment, because I would earn {0:,.2f} {1}".format(profit, CURRENCY_UNIT))


# - Main Function -
def main():
    """The main program that is composed of all the small functions"""

    while True:  # Repeat the inputs until they are valid
        total_cost = input_total_cost()
        number_of_items = input_number_of_items()
        item_sell_price = input_item_sell_price()

        if not is_input_valid(total_cost, number_of_items, item_sell_price):
            print("Please retry.")
        else:
            break

    revenue = calculate_total_sell_price_without_tax(number_of_items, item_sell_price)
    evaluate_profit(total_cost, revenue)

main()