I think it means clean-energy startups that costs multi-million dollars. Clean energy refers to renewable energy and the cost requied to start such ventures are definitely high due to the high barriers to entry and the high fixed cost. It has also been widely reported that countries that uses clean energy more has a higher electricity cost (e.g. Denmark). I agree with the rest though that the context is needed so that the interpretation given can be more accurate. So maybe you wanna show us the paragraph or the statement? :D

The way I interpret the phrase would be to mean that the clean-energy startups cost multimillion dollars. Without a proper context in which to view the phrase, it is all speculation, but I take it to mean that clean-energy startups are not cheap, unfortunately.

To me, it's not clear. It could mean millions of dollars of capital, turnover or profit. I wouldn't understand this to mean the entrepreneurs who make an annual income over millions of dollars though.