Hi can someone help?

My car is burning a great deal of oil. The problem really started when I had the rocker cover gasket resealed as it was leaking oil from there, and after having had this done the oil consumption went through the roof. The garage that resealed the unit say that this was nothing to do with their work. It averages about 300 miles on 3L of oil. Car has done 146,000 miles. No major leaks apparent on the floor. Intermittent smoking. So I took it to another garage that looked at it, and they said the breather system needed cleaning, they then had the throttle cylinder cleaned, oil separater and pipe replaced, but still no change in the oil consumption. This has lead me to believe it is the valve stem seals, Does anyone have an idea of how much these would cost to replace? (including labour). I would welcome any advice or recommendations on where would be a good (value) place to get the job done. Thank you.