You're right, but traditional fossil fuel plants still cost a lot (a comparable amount infact) to build ($2billion for 600MW[1]). Renewables just have a much lower energy output (only work when the sun shines/wind blows, and at a lower capacity) and therefore a much longer payback. But you're right, not having to pay for fuel would off-set some of these negatives.

[1] http://www.synapse-energy.com/Downloads/SynapsePaper.2008-07...