Use s3pe, open up the GameplayData file in your Sims 3 program directory (inside the folder called "Game"), and export the HarvestPlant xml. Create a new project in s3pe, import the xml file you just exported, and try editing this line:
<kCostMultiplierForPlantables value="0.6, 0.7, 0.8, 0.9, 1, 1.1, 1.5, 2, 2.5, 3, 4">
<!--Cost multiplier for harvestables. Format: (Horrifying, Putrid, Foul, Bad, Neutral, Nice, VeryNice, Great, Excellent, Outstanding, Perfect).-->
</kCostMultiplierForPlantables>
Changing all of the numbers greater than 1 to 1 should bring down the prices of produce when you sell them at higher qualities.
Once you've made your edits, save the new project, name it, and place it in your Overrides mods folder.
Not sure if the same can be done for selling fish as there was no reference to a cost multiplier in its xml. It might be something done in the code, which would be more difficult to change.
If you're unfamiliar with making xml tuning mods, here's a good tutorial:
Tutorial: Sims 3 XML Tuning Modding