Bulajap: That's a very common question here, though I think that your $300 estimate might take first prize in the category of monopolistic ripoff! A bit of work with the search function here will get you good directions, and even pictures, that will walk you through a really simple job.
We were captive to the grotesque $30+ price for Subaru's own filter for the first few years, till the aftermarket people decided that we were a market worth investing in. Many of us used to roll our own replacements out of furnace filter material, though the price of aftermarket has now gotten down to a reasonable level and there's not as much of that still going on.
Me however—I've still got enough furnace filter left for 2 or 3 more.
2003 XS Premium MT >200k miles! ● 2005 Impreza 2.5 RS MT ● Robin-Subaru Powered Generator