They look metal to me - they are reflecting parts of the base on them, unless they are chrome painted plastic, which i doubt very much for $500
You are looking at the wrong part the arms not the plates. The plates are likely metal.
Also the price of $500 is speaking of the Japanese market where the g27 is $450. So high price doesn't explain away those cross supports which would mean cast steel parts that are more expensive than folded steel used in the g27. All without offering any structural advantage. //That said the extruded plastic is more than sufficient it just looks cheap.