Tabby - it might help if you told us which country you are in - there may be someone else from that country who knows the prices. The USA prices aren't really relevant as they don't translate in a predictable way into prices in other countries. If however you want to see US prices click here to set your browser to US pricing.
As for the difference between a 272 and 275 brush - there are minor material differences. The fibre in the 272 brush is "Select Goat" and the 275 is "Japanese Pony" but both are designed for shading and blending eye shadow and the shape is the same. The main difference is the size - the 272 is described as "small" and the 275 is "medium" - which accounts for the price difference.