What's the estimate for a starter solenoid, including labor?

Starter solenoids are not generally replaced individually as they were in the old days. Most common now is to replace the complete starter assembly - including the solenoid. Please check the repairpal estimator for the replacement cost of your starter motor.

I have the 98 Legacy Outback 2.5L - The solenoids were aged by a few years in the Las Vegas heat and my shop replaced them at 120k miles with Toyota-made ones (I guess they fit!) and the job came out to $200 in Southern California. For a price comparison, three days wait and $450 for a Subaru replacement, $350 for a low-quality (AutoZone-brand) piece or the solenoids for $200. The starter motor on my model was solid. The trick to see if it is the solenoids is to quickly repeat the key turn until it starts. I learned that you get it hot and moving. I did get a new alternator at 105k as I was warned that the 98 Outbacks have a tendency to fail young, same with the water pump.