Standalone waterfarms are designed to grow a single plant... no more. I've heard of people attempting to put multiples in one waterfarm... but it never works out.

If you're trying to get 4-6oz.... I'd go for a compact multi-site setup (like the smaller AeroJet systems).

It'll be hard for you to harvest that much using hydroponics on a $100 budget.

For that much money, I'd buy and Earthbox and throw two plants in it and grow in soil. That way you'll have a little extra money to spend on your lighting.