https://www.theregister.co.uk/2018/03/06/zx81_at_37/

The ZX81 was launched 37 years ago this week as a £49.95 kit (£69.95 assembled) and introduced an entire generation to the joys of computing, fights over the family television and prodigious use of sticky tape.

Intended as a successor to the ZX80 (£79.95 as a kit and £99.95 with all the soldering taken care of-ish), the ZX81 improved on the previous model in several subtle ways.