Am surprised that Curviloft is so expensive.
It is £55 a year. This is about one-ninth of the cost of buying an open license for SketchUp Pro 18.
I would not mind buying a one-off open license but there is no guarantee that I will use the plug-in very much so I am reluctant to pay so much for just one year's use.

Can someone design a more cost-effective plug-in to do the same job?