Each material has a price and for that reason, anyone could judge if it's fair or not. If an aluminum enclosure for storing the batteries, plus some electronics and plastic costs too much, it doesn't have a fair price. It's possible to find enclosures for 18650 standard batteries, especially designed for e-bike conversion with a price near $80 / 45€. Some of the links are from China, as the market there is great, as the use of e-bikes.
Search terms: “18650 enclosure ebike”, “18650 ebike”
With these options, there are little possibilities to consider luxury a battery pack for an e-bike conversion kit, even with high capacity.
The most expensive part should be the battery pack, thus with the motor and the throttle mechanism cannot add enough to surpass $250-300 / 180-220€. When in some places there are bikes with acceptable quality for everyday use at about $245 / 180€, the total price would be well below the usual one for e-bikes. The charger could add up a little but the battery enclosure should accept standard chargers, and there's who has more than one lost at home.
With Do It Yourself policies sometimes it's possible to achieve the best option, while you learn topics that might be very useful for countless activities!
An electric bike perfectly serves for substituting the combustion vehicle. Did we mention that with solar panels the energy is completely free and doesn't produce carbon dioxide?