That's a great question!
In our neck of the woods we have a number of repair shops that show standard rates to repair head gaskets starting at $1,400. In the description of this repair it states that this "doesn't include milling" and "timing components additional". So, by this reasoning, you'll be doing a major repair without doing any of the truly important work that should be considered with a repair of this magnitude. This is playing to the idea that by saving some money when doing this work one can simply kick the can down the road and hope for the best when something else fails.
Let's start with why this is not a good idea by first speaking to what a head gasket is and why it is so expensive. The head gasket is the main seal between the top and the bottom of the engine. It surrounds the cylinders of the engine and lets oil and coolant cycle around those cylinders to keep things lubricated and cool. The issue occurs when the material of this gasket fails and allows oil or coolant to start escaping from outside the engine or, worse, into the cylinders themselves. Either of these circumstances can, and often does, cause engine failure. This is most likely to occur when an engine becomes overheated, but in the case of the Subaru and the way in which the engine is designed (where the top of the engine is really the sides of the engine) it can occur as just normal wear and tear to a greater degree than just about any other make of vehicle. Thus, we have the dreaded "Subaru Head Gasket Failure" that is all over the internet. When the engine overheats it is very common for it to become so hot that the surface of the head or the engine becomes warped which requires a "milling" - a way of making sure that a flat gasket will sit on the engine and heads without any gaps.
Now, let's return to the first point that I made at the beginning of this article about milling being an additional cost. Why would anyone put a flat gasket on an uneven surface? The thought is that the layers of the gasket are cushiony enough to fill in any idiosyncrasies that might have occurred with the failure. Talk about rolling the dice! The likelihood of future failure is near imminent. There is simply no reason not to mill the surfaces unless, and this is a big "unless", the head and engine have already been milled before and there simply isn't enough meat left to mill without the engine pistons and valves running into each other. Even then, it is vital that a flat edge be employed to assure that the surfaces can sustain a gasket-only replacement.
As far as timing components go, think about this: to do Subaru Head Gasket replacement, the complete engine must be removed from the vehicle and all timing components taken off. Unless you just did timing component replacement less than 30,000 miles ago, there is simply no reason not to put new parts on there. When reusing the old parts you are again just praying that they hold up instead of having the assurance of unflawed new parts that, by the way, come with a brand new warranty. In fact, when head gaskets are replaced it is common sense to replace any engine parts that come off and that aren't like new. This can include spark plugs, ignition wires, thermostat, oil pressure switches, variable valve timing gaskets, oil pan gasket, etc. In this case it should be a "one and done" repair. You may as well seal up every leak on that engine while it's out of the car.
Now, what's the cost? Usually for our shop this is around $2,400. While this may seem like a lot, just consider the difference between flying Frontier Airlines and Southwest Airlines. Frontier will sell you a bargain basement ticket and then charge you for everything under the sun that you would want when flying (comfortable seating, baggage, snacks, good service) while this is standard at no cost on Southwest. If you're going on a vacation you want to actually enjoy it, rather than just get there with a la carte services that usually end up costing the same as the Southwest flight anyway. The same theory applies here. Do it right for the price that is fair and accurate rather than a piecemeal job that will leave you wishing you had spent a little more to be worry-free.