I've tried Life Extension, Walk-in Lab, DirectLabs, and DiscountedLabs for one-off tests, and have been happy with them all. Depending on your insurance, they could even be cheaper on full panels.
I noticed I was getting gouged by pricing differences between providers, so I made a (very-incomplete) comparison site. Here's the pages for the tests you mentioned: B12, homocysteine, magnesium (RBC, serum). Feel free to let me know if there's any others you're interested in comparing, it gives me a reason to not be lazy about improving the site :)
The Life Extension sale that ta5 mentioned is usually really good, ~30-50% off from what I recall. And both Walk-in Lab and DirectLabs usually have those 15-20% coupons every 1-2 months.
B12 EUR 14,57, homocysteine 20,98, whole blood Mg 2,68 at a local lab. Usually I get as many through my GP with a referral to get the blood draw done at the lab. Then I add and pay things out of my pocket which my GP wouldn't.
I take it this is in Austria? Those prices are amazing. Is the doctor referral required? Or could you just go directly to the lab and order out-of-pocket?