i think the new 100 macro with IS is somewhere around the £800 mark. The problem with IS and taking pictures of insects is that it will keep the lens elements steady for longer shots but you have still got to contend with wind movement and the insects moving.
To get a good depth of field you need a narrow aperture. I use a maximum aperture of f/16 for insect photography and quite often f/20. This gives a good depth of field but means the exposure is far too long to hand hold. A tripod would overcome this but you then have the movement problem again so I use a Sigma ring flash and manually set the exposure to 1/125 at ISO 400. This combination gives a good depth of field with a nice sharp image.
