Wall cavity insulation is a wonderful way to save money and make a home much more comfortable. In a retro-fit case a specialist can drill holes abouty 1.5" dia.at the top of each cavity and blow in a pelletised cellulose or fiberglass fill. The blowing process is great because it fulls all voids and reduces draft and infiltration. The insulation itself is very cheap but all the process and patchwork can become expensive, depending on your circumstances. But bear in mind that the roof insulation, window and door fit, cellar and roof perimeter sealing, are vitally important, so the best walls in the world won't make much difference if the rest of the house is not improved as well..