Well, even a cheap Raspberry Pi is £30. Add in a decent power supply, SD card, and case - you're up to around £60. Add in a good WiFi module and that's £70 - more if you want one with an external antenna (useful if it's installed in a loft).
Hardware wise, that's probably all you need (plus a cheap USB or serial cable).
So, now you need to write some robust software which will continually log - and recover in the case of power failure. Also needs to have a good looking front end. Will need regular updates to deal with emerging security threats. Oh, better make sure the case is hermetically sealed so all the spiders in the loft don't make their home in there, die, and short out the circuits.
Finally, you'll need a web server for external logging - which, again, will need constant development and maintenance.
All of a sudden, that £400 looks fairly reasonable for a "plug and play" box with good set of cloud services.