Blown in dense pack cellulose is a good option for places where you don't want to disturb the inside walls. It is applied through ~2" holes from the outside. It is one of the cheaper forms of insulation available, and very effective. Depending on your siding material they will pull one clapboard or lift individual shingles to make the holes. Worst case they save the plug and put it back with some caulk. Generally not very noticeable with some touch-up paint. At the same time they can give you as much blown in as your attic will fit. You will want to make sure that the soffit vents aren't blocked.
Your utility company may offer a program to help offset the cost of insulation, doesn't hurt to ask. (In MA, for example, you can get $2000 worth of insulation per year for approved projects.)