First, where did you get your
GS from? If you're buying the
GW pack, you are right it is monsterously expensive (my experience leads me to believe that
GW only really wants you to use it for gap filling and minor sculpting work on conversions). Try looking online: heresy has some in stock now I believe (
http://www.heresyminiatures.com) or you could look at eBay. You can get tubes or ribbons of
GS for ~£10 which are about 4-5 times the amount in a
GW blister. You could also check out your local Hobbycraft as they (should!) sell the GF9 tubes of green stuff and grey stuff, BUT it may be more expensive than buying online.
For bulking out armatures you could try using milliput - it's cheaper, is water soluble (which makes smoothing it down really easy!) and drys rock hard so you can scrape/file/carve it into the shape you want.
I would also recommend getting some clay shapers. Size 0 flat chisel head and tapered head are probably the most useful to you initially as you can use them for both mechanical shapes (armour, etc) and organic shapes (faces, clothes etc). I assume you have some sculpting tools already - hopefully not just the
GW sculpting tool! If not invest in a decent set (not necessarily the £25+ sets, but not the really cheap ones either).
Get yourself some reference pictures of anatomy, musculature, clothes - these are invaluable if you are trying to make a convincing, detailed mini.
Finally - PRACTISE, PRACTISE, PRACTISE!!! I really cannot emphasise how important it is to keep trying. It is fairly unlikely that you will be brilliant at this immediately but the more you try the better you will become.
Hope that helps
Stuart