Cost of a blown head gasket

[bugsntinas 29]

As some know my car over heated and had fun and games with the thermostat. I still haven’t bothered my . trying to start it but if as a worse case scenario the head gasket has gone what is it likely to cost as if I sell I need to take that into account

[sheffieldcortinacentre 750]

If you can do it yourself price of a headgasket (£20 ish & a steady afternoon.