Filling My Holes

I was looking at Fox Farm Ocean Forest potting soil because it has the best reviews and all the stuff a marijuana plant needs, but it's super expensive. If I dig a hole 1.5 ft x 1.5 ft how much cubic potting soil will I need to fill it? I'm assuming for a hole that size I'll need 1.5 cubic feet of Fox Farm potting soil, but that's really expensive because I want to do 30 holes and that would come out to be nearly $800. I was wondering if mixing it with the pre-existing soil would be a good idea to conserve money or if I do this will it hurt the plant's yield or potency any? And if I do this I'll be adding fox farm nutrients to the soil according to their feeding chart if that helps you to answer any. Thank you in advance for any help I can get on this question because I'm in a dilemma right now.

This thread will recieve allot of views on title alone...

Assuming your hole is a cylinder v = hπr[SUP]2[/SUP]

You need 2.65 ft[SUP]3[/SUP] to fill your hole.