As for your question - if the seed is cheap, I think it's better to buy it, it's hard to do that when we get the quest to plant 4 Eggplants :)
You will need these cheap seeds in the future anyway.
If you don't want to buy all the seeds of a season in large quantities at once, the quest quests give you a clue as to what and when to stock up.
Another thing is that in order to get down to doing the quest at all, it would be good to have a minimum supply of crops from previous seasons.