That's a good question.

Short answer: I don't know.

Long answer: I'd hesitate to invite government regulation because that ruins everything. You'd probably want it to be just enough money to incentivise continuing the pregnancy, but not enough to want to do it all the time. But to be honest I'm not sure people making a career out of it would be a bad thing. If there is a market demand I don't see anything wrong with being a professional womb. If people are buying it's a win-win. The problem is when the state gets involved and starts handing out ludicrous amounts of money and ignoring any market dynamics; thus creating wards of the state. I imagine supply and demand would go a long way to regulate the price.

I suppose we also have to think about it from a buyers perspective. What would I want? I'd want a trusted third party managing it (surrogacy business). I would want regular drug and health screening. I'd want genetic screening. I'd want the price to reflect the surrogates commitment to a clean bill of health, etc. I think this would go a long way to making it painful enough for most people to not want to make a career out of it, but enough of a fall back if you had an oopsie moment.

I also think health insurance could help here. I don't know about most people but I'd like a policy that gave me the option to get paid out to continue a pregnancy if I was raped. Sounds a bit full on, but insurance for trauma is never nice to think about.

On a side note the rape argument has always confused me. Abortion is still immoral when a person is raped, but the moral responsibility lies with the rapist. If abortion is chosen then the rapist is responsible for the murder, not the mother or doctor. I think states that make it illegal have exceptions for rape anyway.