There is no general answer. It depends on the local solar and wind resource, the match between demand and renewable supply, the penetration level of renewables, the organisation of the market, the presence of (hidden) subsidies on conventional power supply, and environmental laws.
Some points in more detail:
In arid hot regions where people have a lot of air conditioning, the diurnal pattern of solar power will match the demand very well. Up to a certain penetration level the solar power will replace expensive peak power, resulting in lower overall costs.
If there is a free market for power production the merit order effect can reduce overall prices on the spot market, reducing the profits for conventional power plants and reducing cost for power buyers. This happening in Germany, unfortunately these cost reductions are not passed on to costumers.
( see for example: www.isi.fraunhofer.de/publ/downloads/isi07a18/merit-order-effect.pdf)
In many regions wind energy is competitive with new coal and new nuclear power plants. In New Zealand wind energy is competitive with existing conventional power plants without any subsidies or support.
The cheapest conventional power plants are mostly old and dirty (fine dust, NOx, SOx). Strong environmental laws will force these plants to close down or to install costly refurbishments.
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