At one time there was much discussion of this system on the Mach 3 group. In fact Mach 3 has the facility to put the image right on its own window, without added software, including cross hairs.
The offset can be taken care of by writing a simple VB macro and attaching the macro to a screen button,so it really doesn't matter how weird the offset numbers are.
There are also systems that combine lasers and a camera.
As you already said the results depend on how much you are willing to spend on the "stuff".
For me it seems a better approach to use Mach 3 's probe facility, G31 I think. There has been quite a discussion of it, I think somewhere on this site. I'll try to find it for you. With some simple probes that you can cobble yourself for no money, you can obtain near "dead on" accuracy for edges, centering over a point or feature, centering within an existing hole, and even tool height.