Bryan
According to
this recent post it can be even cheaper than what I've listed. Oxycheq's El Cheapo is $100 inclusive of O2 sensor ( which works out about £55), whereas if you buy the two units I listed and build it yourself, it comes out at about £80.
I guess it's all down to whether you have the patience to do it yourself or just want to unwrap it and off you go.
Anyway, either way, it's kept me busy for an hour this morning rattling on about it!
Regards, and dive safe
Martin