Inflation rates have to remain kind of as they are. Otherwise the late game gets swamped with gold.
The main question has to be answered first - are Corps supposed to be spread arbitrarily like Religion, or is there supposed to be an important decision making process to decide where Corps should go??
If the latter, then the problem is much more complex and what we have now might not be too far from what Firaxis intended. Take the food Corps for instance. I have a food heavy city which has reached it's maximum land working population. Every extra citizen after then is therefore a GP. Add a food Corp and this city just became the greatest GP factory in history, and this is in the late game! This simply isn't possible under SP. Not only that, but bolstering backwater cities with production or culture Corps can transform them in an instant into really useful and competitive cities during the late game. Again this flexibility isn't possible under SP. If this is the case then my main concern is the AI, which seems to spam Corps (foreign Corps at that!) among all their cities. Either they're getting some hidden economic boosts somewhere, to combat the inflation and costs, or the AI just isn't up to dealing with the choices that having Corps (particularly foreign Corps) enforce on you.
If it's the former, however, and spreading them is a no-brainer then Corps are far from what Firaxis intended.