The popularity of Japanese erotic games does not rest solely on the game's concept, story or illustrations. The actors who lend their vocal talents to the game are important, too. But they're not free!
According to Japanese bulletin board 2ch (via website Canned Dogs) the average rate per word is ¥100 (US$1), and popular voice actors earn ¥130 ($1.39) per word. It is possible to get the lowest rung voice actors to read for ¥90 ($.96) per word. There are extra fees to negotiate certain types of scenes and whatnot.
In most cases, the price-per-word will not go down if a voice actor has, for example, 5,000 words. However, if the part is less than 100 words, the voice actor's agency might not accept the job due to transportation fees. It is also possible the agency will ask for a flat rate of ¥20,000 ($215).
Some voice actors will not allow certain words like *ahem* "chinko" ("penis") or "manko" (look it up) to appear in the game. In this case, it is necessary to edit those words out of the game. Some voice actors even get a percent from a downloadable version, a budget version, a PG version, or console ports.
The reason why erotic anime have different voice actors is that the game voice actors often refuse to do the voices. The reason? Erotic anime apparently only pays ¥10,000 ($107) per episode.
It is noted that in the past five years, voice actor recording costs for adult games have increased anywhere between 150 to 300 percent! What's more, five years ago, voice actor agencies did not charge per word or ask for secondary use license royalties.
Pictured is veteran erotic game actress Ami Suzuki, aka "Dynamite Ami". That link might be NSFW, depending on where you work.