They are very expensive. ~350B, plus the extra 100B penalty for buying a corp with market value > 200B, equals $450B purchase price. A CEO can expect to earn at most 1B per month from a well managed cement corp, so that's 450 game months to earn back the purchase price. At 6 game months per RL day, that's 2 and a half RL months. Personally I prefer to invest in things with better returns.
If you are anxious to get those corporations sold off, you might consider bringing down their value by keeping their production off the market for a while.