We understand that when times are difficult cost is important, but so is good posture and comfort.
Here is an idea to give you a full looking cushion (see more images tab) at very little cost. The solution to you problem is:
SHREDDED FOAM CRUMB.
This method work we for seats or backs
Its not an exact science but one of our 8 kg bags of mixed crumb is enough to fill 2 average DFS cushion
Method
Open zip, remove old filling, tip in new filling, close zip. JOB DONE!
It really is that simple and very easy to maintain. The more you stuff in the firmer your cushion will be.