That's certainly a bit of a conundrum Arfa, the price difference is massive?
You would like to think that a company like M&P would not mug you off by selling you the £9 kit that could in theory be sub standard to OEM, but in the same breath they're just rubber rings, could they really be so very different?
There is another route you could take, but it's taking a bit of a chance, and that is to buy the £9 set AND a used pair of calipers, which will probably still come to less than £100.
This way you can have a set to clean up properly with the bike still on the road, and if the seals don't work out then resell the calipers.
You could be lucky and buy a perfectly functioning pair of calipers which could be left alone and fitted straight away, then reseal your old set as a quick spare for a quick change over when needed.
Just a thought.