Enviro corps only costs 25% base cost more than Free Market corps. Eviro corps cost net 28.5(ect)% more than Free Market corps.
The games mathematics are less than elegant and they really get messy with corporations. The gross Civics modifier are calculated before the courthouse modifier, so their final effect of civics is only net half of what they appear to give (12.5%)
Free market doesn't give you much late game unless your tiny, and if your tiny you probably don't need cheap corps anyway (not enough resources for them to run a big deficit). Unless you own less than 20% of the world that last trade route will probably be a weak internal one in most cities, and will earn you less than 1 enviro windmill. Free market can be good when you first get it, but it gets weaker every time you get a new trade route, ironically corporation really weakens its use.
Corps have to go pretty crazy for that 30% bonus to outweigh all the environmentalism bonuses...
It's also less important in my playstyle, since I prefer to run Civ Jewlers\CreateCon\(rarely)Alumco rather than Mining Inc. Civ Jewlers will typically pay for about one other corporation, and if Cid's Sushi is really going out of hand than Alumco will pay for it and give beakers AND hammers and culture via Createcon. The corps don't sink me because I balance them out.
Plus if you ever want corps to really shine you need to enviro lock everyone via the U.N. anyway, or any AI you spread your corps too too much will just lock them down via civics. Once everyone is enviro locked your corps will sink the AI while making you rich no matter how expensive their individual maintainence is.