$10.50 a liter!! What fluid is specified for your transmission? Mine requires Dexron III ($1.15 per quart).
It sounds like you get at least 90% fluid change in your transmission. I wouldn't recommend using the flush system I described.... probably wouldn't do any better. I use the flush system on other cars only after a filter change and only because dropping-the-pan method exchanges no more than 30-40% of the fluid.
95 E320 Cabriolet, 131K