That issue is very repairable. Could be as simple as making sure the sparkplugs are on tight enough. But more than likely, happens on many cars, the gaskets that seal the sparkplug tube on the valvecover to the cylinder head are leaking. That could be caused by a number of things - age, type of oil used, oil change interval, if it every overheated, etc. - but is easily remedied by just replacing the gasket.
The gasket itself is inexpensive, most of the cost will be involved with labor. Not sure how much it would cost (time-wise) on a Subaru, but on most Toyotas - its about 1/2 hour to an hours worth of labor.
2002 Corolla S, Silverstream, 1.8L 1ZZ-FE (VVT-i)
2003 Matrix XRS, Cosmic Blue, 1.8L 2ZZ-GE (VVTL-i) - RIP
2009 Matrix XRS, Blue Streak, 2.4L 2AZ-FE (VVT-i)