It seems like a lot to pay for a 'bit of foam', but I've tried many of the Airex alternatives found them all to be either too soft or too hard.  There might be a version available for a fraction of the price, but I reckoned I'd spend more money searching for it than a 'real' Airex costs.  So, I bought it.  I've used one many times at the gym, so I knew what I was getting.  Its great for balance exercises and some stretches I find essential after a day in front of a computer.  Yes, the one I bought is likely to start cracking and flaking and need replaced.  But, until it does I'll be using it most days.