if what you have is enough, that will -as they say- "keep body and soul together"; then run with it!- there is time to upgrade when finances allow, biggest/best thing is to get 'out there'!
You can take a ccf pad and punch holes where you want the wings, buy another ccf pad and cut out wings with holes punched in them to mirror the main pad. Put the pieces together by tying line through the holes on the pad and wings, and you have cheap under insulation. You can add grommets if you're feeling confident, and want to use this for an extended period of time. My 30 degree down bag and ccf have gotten me to 30 no problems... this is all, however, my advice only if your deciding factor is cost alone (which it seems to be).
What saves a man is to take a step. Then another step. - C.S. Lewis