Best Budget Grow Ideas?

Ok so I have a budget of around $550. I already have a 600w hps from a previous grow and a 4x4x7 tent. Now what are some ideas on stretching this budget to making this a succesful grow? Im up for any good ideas! Thanks in advance!

Hydroponics

Before you buy anything else, I would get a carbon filter, if you already don't have one.

With the money left over I would get another 600 w light, more light=more pot.