MY apexi set me back more then 30£ to be honest... closer to 100€ If I'm not completely wrong, due a change actually. I got mine in Ireland, I'm sure one of the bigger traders or smaller could source them for you... if they don't just put you onto their own brand that they sell? Maybe worth a gb?
I change mine when I 'feel' it's due a change, look at it first and bang it against the wall and if it's lots of particles falling off it I either clean it with high pressure air or bin it. It's all to much of a hit and miss to say every 1000 miles etc, depends to much on your driving and where you drive...