Do anime with more ecchi scenes pay their seiyuu more than non-ecchi anime?
From what I have read, eroge voice-work is not a desirable path. The pay is less, although I'm not sure how much exactly. I think voice actors are generally paid by the word, and long/complicated lines are not usually required in eroge. I don't know how they would measure a payment for just breathing and other sounds.

Most voice actors are similar to celebrities, in that they want to become well-known and gain reputability for their skills. This will give them higher paying jobs in more popular series, and their demand will grow. However, eroge work is probably viable if you need to make money or start out. A lot of well known voice actors have done eroge work before becoming more established and moving away from it. The voice actor for Vegeta in DBZ apparently did eroge at one point. A lot of "voice otakus"? will try to match eroge VAs who used pseudonyms to their real names, which is a deterrent for some VAs to even do any eroge if they care about that. 

I am sorry I don't have any real sources to any of this, so take it with a grain of salt. I've just googled through some message boards about this topic, like the one below

Url may contain content not safe for work

http://blog.livedoor.jp/myonkui/archives/5522517.html