OEM is now Vertex. Last Vertex kit I bought in winter '07/'08 was around $130. I hope those prices are a mistake, $300 is just stupid expensive for a two stroke piston.
The Vertex piston is and always has been the same for a TM250, and I beleive is listed as a standard Vertex part. Call around, there has to be a dealer with a 250 piston in stock for a reasonable price. Failing that, if that $300 is for real, try getting one direct through a Vertex distributor (Sudco I think).