You're right, but it strongly depends on where you're living. For Europe it is not much, I agree.
What do you mean by BD-R "M" XL, the Millenniata version? I don't think that they are so special as they claim to be. They also use inorganic materials for the data layers, as ALL HTL BD-R media do. (For DVD this is different. All DVD-Rs except the M-Disc use organic dye material. But the M-Disc uses inorganic material. They might age slower, but the initial burn quality is also very important....)
You're absolutely right!!! Also here in Switzerland. For a 5pack Verbatim BD-R XL 100GB I paid about 8.50 Euro/Disc, where as for the above mentioned 10pack (VBR520YP10SV1), 1 disc cost me about 4.50 Euro (and that's incl. postage which is about 1/4th of the price!). The quality seems to be the same (both have MID: VERBAT-Imk)