The mat system is brilliant if you can afford it and are lazy. if you are on a budget though try the single wire system from wickes DIY , it just takes ages to set out and get the spacing right but once its in, works exactly the same. It also comes with a under floor thermostat probe so no problems with burnt feet!!!!

As regards the cost to run, 1000w per hour is one unit (1kwh) So 250w of heating would use 1 unit in 4 hours or a quarter of a unit per hour. But bear in mind it will not be on perminantly as the thermostat will click it on and off once up to temperature. My kitchen has 600w of heating and not only have i got a toastie floor but i dont even need the rad turned on cos it heats the room as well. Also be warned it takes ages to get to temperature once first swiched on, so dont panic if you think its not working. Hope i havent confused you even more.