Well some things in Rune Factory actually sells more in a seed bag form than the actual product. Which is somewhat funny at times. Like for example, the normal level 1 price for a seed bag is 80 G. when it's level 10 Seed Bag, you sell it based on this calculations:
[Price of Selling Item] x [Level if item] = [Total Price]
With that said, there is times where sometimes the crop fully grown makes more money.
Putting that aside, I think you should check again if you are looking at the right prices. If I were to sell a seed bag for Pink Turnip at Level 10, I would only get 400 G and selling the Pink Turnip at level 10 will get me 2400 each. So not sure where you getting the prices from or how you doing the calculations now.