I have to agree with Luc's great suggestion of the mach wizards, but then since I wrote and support them I am slightly biased
However, they are conversational wizards, not a real CAM system. They are fine for common shapes, like circles, rectangles, pockets, bolt circles, arc slots, etc. The set called "addons for Mach" were written to run under Mach3 and cost $50. The new nfsMill set is standalone and will run on any windows system. They are $75. They are kind of the quick and dirty approach, very simple and fast to use but less capable than a CAM system.
In full CAM software there are several good packages in the $200-300 range. I have used Sheetcam for several years and it has done very well for me. It is really aimed at the router or plasma guys, hence the name sheet. I have used it to make many parts for model engines. It reads dxf files and generates gcode.
There are several very good products from Vectric. Cut2d and Cut3d are as the names suggest. Their interface is similar and work well. As I recall they are about $200 each. Vectric has a more expensive program, Vcarve pro aimed more at the sign makers, but I have used it for model parts, especially name tags and labels for boiler and locomotives. It is about $700, and would not be my choice for simple model work. I have it because they were at one of the early CNC workshops in Galesburg IL and offered a package deal.
There is also CAMBAM which is highly regarded. I have used it, but only in demo mode- it lets you run a limited number of sessions, I think 40. It is probably the most capable of these, but a bit steep on the learning curve. I think if I were starting fresh today I might pick CAMBAM because of its more advanced capability.