a normal doujinshi (around 20-30 pg) is around 660 yen ($4.4) a long doujinshi (around 40 pg) is around 880 yen ($5.9) a manga (180-200 pg) is 1100 yen ($7.34) Colored works are generally 30%-50% more expensive than the B&W version.
A few artists (and foreigners like Chinese or Korean) will charge more, but generally Japanese artists will stick to this standard. Also note that many of these regularly go on sales, so their final cost maybe cut down 60-70%..
so yeah, it really depends on how confident you feel about your work. As for me, I just ask if the works in question are worth X amount of manga that I can buy instead.