Unfortunately it depends if you mean now, or a few months ago. It performs similar to a 3500X, which should (it isn't) be priced around £115 - £130 new, so I'd say under £100 is reasonable, but £70 - £90 is a good deal.
I'd want the 1660 to be under £200, because the 1660 Super was available around £220 new not so long ago.