How to get corp value up for IPO?

I've been trying to IPO one of my countries for a few RL weeks now, set the taxes to zero and waited for the corp values to get up above 500B so I can sell them off. But none of my corps are worth more than 300B, and I'm not sure how to drive their value any higher. There's a wide spectrum of corp types, including several that are generally pretty profitable, and I have a welfare of 129, so I'm not sure what else I can do here.

Any tips? The country is The Barren Barony on WG, if that helps.

It is a bit counter-intuitive but you need to drop your profit sharing to zero also, since value of the corp is figured from net profit rather than gross. Setting your corp supplies to around 210 would also be more optimal and I would recommend setting your sale strat to "best price" to ensure your sell price isn't under cap.