Hi Komi!
Well for the ones who live in the USA or maybe in Canada it may not be that difficult to get all at reasonable price. A few weeks ago I saw an Amazon offer which had the volumes I am still missing. Price would have been about 7-10$ each, but the international Amazon shipping cost made the total too much. Shipping would have been 12,49$ x 3 and it was not possible to combine the shipping cost even though the mangas were all from the same seller. When I got interested in Kindaichi, I should have waited a good package offer. Now buying manga one by one is too much for my wallet.
At the moment my country only offers volumes 5, 6, 13, 16, 17. When those are sold out, there won't be anymore Kindaichi. I also have been using
Bookdepository.co.uk. They mainly offer the recent last two volumes. In addition, Bookdepository is selling some Kindaichi volumes at the price of 17 euros. Publisher is said to be Paw Prints, who is that?
I think the first six and the last two are easiest to find at a good price. I would say, especially the volumes 9-12 are the most difficult to obtain. Luckily, I bought the volumes 10-11 when the local online shop had those. Actually, I got the last pieces.
I know about Kodansha's bilingual manga. Those are also rather expensive as you pointed out and nowdays even more difficult to get. However, I have the first four volumes. I bought those because I thought they had a new story as it's named The New Kindaichi, but in the end it's the same story as in TOKYOPOPs manga. Here is a few images I took (sorry for the quality, can take a better if needed):
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Cover & Back
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An example panel
I am very fond of these. However, I would rather read the new cases of Kindaichi. So, if you're interested in exchange and have volumes I am missing, I could negotiate about exchanging. You probably can get TOKYOPOPs editions back easier than I. PM me if you're interested and I give you my MSN/email address or I could directly contact your email if you let me have it.