How much would it cost to repair a head gasket on a 1997 Subaru Legacy Outback

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I think my head gasket is trashed. I had a flat tire 2 blocks from home, so I slowly (meaning < 5 mph) drove it back to the parking lot in my apartment complex. As I pulled into the lot, I noticed black smoke from pouring from the rear of the vehicle. I don't want to start the car because it may cause further damage. If, by chance it is a head gasket, can someone give me a rough estimate on how much it would cost to repair it? Or, would it even be worth it? I revisited this site because the folks here provided me with no BS answers when I had to replace my clutch kit about 9 months ago. In realty, the advice I received saved me hundreds of $!!! I was able to find someone to replace the clutch for $550--PARTS AND LABOR. Granted, that's a lot of $ for someone as poor as I am, but considering the difficulty of the repair, by comparison, it was a fair price. Local shops wanted to charge me from $900 - 1,500 to replace it.