I have the Tommee Tippee ultimate monitor which cost £100 from Tesco, but that was a few months ago when Damien was about 5 weeks old. I think they still retail at £100 on the Tomee Tippee site though?? I think Argos sell the ones which Tommee Tippee don't make anymore, hence they are so cheap. They had one a few months ago at half price for only £25, and Tommee Tippee don't make that one anymore either.
The mats are great for peace of mind and reassurance. My son does actually suffer with sleep apnoea (when they stop breathing for too long, 5-10 seconds is normal) and his mat has been so useful to us and alerted us a number of times when he needed help.
If you do buy a mat for breathing monitoring, I would ask you Health Visitor (when baby is born) or your midwife about baby resuscitation so that if by some awful chance the mat alerts you to a problem, you know what to do an how to revive and get your baby breathing again. I could tell you but I don't want to get it wrong and I'm not qualified to tell you.
Sorry waffled on a bit there!! The mats are great though, and really help to reduce the risk of SIDS/Cot Death.