Depends a lot on where you are at the moment and how far you are prepared to travel to get cheapstorage..
I can name a couple in the Western Med.. Almerimar and Agua Dulce.. ashore on afloat.
If Central maybe consider some of the b/yards in Malta.. further East I will leave to others more knowledgeable.
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In eastern med, Greece probably is currently the cheapest place to go if the price is the main concern. You can google and find many marinas there. If you have some works to be done on the boat, that's a different matter.
In overall, eastern med should a somewhat cheaper than the west, if this is an option for you, as Boatman said.