Here's my money savers - not so much tools but useful bits and pieces for detailing:
Foil seals from Milo tins - it's cheap and a little bit lasts for a long time. It can be cut and bent easily, great for detailing, making straps, seat belts etc. In fact you can make whole panels from it as rivet impressions can be made no sweat (for example, I made the panels between the cabane struts on my BE-12b from this foil, as well as the intakes on the cowling).
Another great one is fine copper wire which you can buy or steal from the windings of slot car motors - great for detailing engines, cockpits etc!
Fine solder is another good one for tubing - bends very easily and holds its shape nicely.
Elastic core from "knitting in elastic" makes great rigging (inside and outside the aircraft) and one small roll lasts a very long time.
Those are some of my cheap and nasty modelling tricks!
BC