I have around $1500 - $2000 invested in my @ 800 gallon ap system. I figure, with all the organic raised food, tilapia and veggies, I should have returned my money within two growing years. You can see some of my design and setup here @ Lordshandyman Aquaponics System v2. or view my post here @ Help, I caught the aquaponics bug...

As for your question about how much to spend, your wallet is the limit...