Well Vcarve and such are also great for one-time purchase versus monthly.
However, they are not machinist level packages....if you need to reverse "engineer" parts, then they might not work. If you are simply trying to get old
signs scanned and recut then they should still work ok. Efficient 3D milling is an expensive nut to crack and most programs dealing with a scanned file are going to only have simple toolpaths. For the fastest production work, yes, then you need better CAM. For
sign work, you generally don't.
However, I would assume that you get a little better CAM just by being "in" Rhino, whereas outside CAM might be limited by the export file type. But you easily could be trying to ask for an improvement in 5% of less of your work while hurting the efficiency of the other 95%.
Other 3D oriented, low end, one time buy software I have used is Meshcam..... without really trying Aspire I think it might be a little more flexible in tool pathing and Vcarve + meshcam still cheaper than Aspire and all the other higher end options. It's what I would look at first if I wanted to get into full 3D
sign work.
There are others too, deskproto for example has a limited rhino integration.