> Doesn't this mean that solar/wind are insanely lucrative?

This is how markets are supposed to work. It provides an economic incentive for production to increase, which is what we want.

Consider what happens if you develop a farming method to produce potatoes for a fraction of the usual cost, but you can only meet 10% of total demand at your local market. What price are you going to sell your potatoes for when you show up to the market? You (like any free market seller) want to maximise your return, so you'll be able to sell for a fraction under the previous market rate, undercutting everyone else. Your farming method would be extremely lucrative.