The valve seals are worn in my 240D, and the consumption is less than 2qts per 3000miles or 2L per 5000Km. Your consumption level is not really consumption.... more like a steady flow of oil out the tail-pipe.
$500 for a valve job sounds a little high. Check around for better pricing, such as a good machine shop. The one local shop to me, did a complete valve job and re-surfaced the heads on my Mustang.... $20 per valve (R&R) and $40 for re-surfacing (including valve seats).... total job for V-8 was $360.... the car runs like new now!
76 240D (W115) - 550K miles
78 300D (W123) - 200K+ miles