The price of porcelain tiles varies enormously depending on the seller and the quality of their products. At their very cheapest, porcelain tiles might cost £15 per m2; at their most expensive, porcelain tiles can cost as much as £100 per m2. Those are the minimum and maximum price points, but most purchases fall somewhere in the middle of these two extremes.
If you're looking for good quality at a reasonable price, you should expect to pay £25 to £50 per m2 for indoor porcelain tiles, and £35 to £60 per m2 for outdoor tiles.
Pictured: 10mm Quartz Twilight Porcelain Tiles from PrimaPorcelain
Thickness is quite a good indicator of a tile's overall quality (although there are other factors to consider, such as grip and the number of different 'faces' a product has). The thicker the tiles, the more you should expect to pay for them; outdoor porcelain tiles generally need to be thicker and harder-wearing than those designed for indoor use, so building a patio is likely to cost more than tiling your kitchen floor.