I'll preface this post saying that I buy a lot of manga. However, the problem is, as with most things, price. A volume of manga from tokyopop bought in Waterstones costs £7. I beleive Tanoshimi manga to be £6, not a whole lot cheaper. Bought online, you can still expect to pay at least £5.
Is it worth it? Now, depending on the style of manga, it takes me between twenty minutes and sixty minutes to read a manga volume. That's between 8p and 35p per minute of enjoyment. Compare that to the most recent book I bought, a new copy of Raymond Feist's 'Magician' (my old copy has literally disintegrated due to being read too much). I paid a whopping £9 for it. However, it takes me at least seven or eight hours to read it. That's a mere 1.9p/minute, about the cost of an average landline phone call, and over four times better value than the best value manga.
Of course, these statistice are horribly inaccurate, being as they don't take into account the fact that I tend to re-read everything, the fact that there is relative levels of enjoyment (some manga I enjoy a lot, other manga I continue reading literally because I don't want to leave it half-way through.)
Will this stop me buying manga? Don't be silly.
P.S: It seems like a huge difference in manga reading times. I'll just say here that the manga I read fastest tends to be things like Battle Royale, with it's huge panels, limited dialogue and abundence of double-page spreads. The manga that takes me the longest is probably Love Hina, with it's tiny panels, massive amounts of dialogue and virtually no DPSs.