Broad answer to broad question is nowhere or everywhere..
"cheap" relative to what, what are yr minimum requirements in terms of safety, comfort, technical capacity, easy flight connections, in Schengen zone or out, will you be living on board or just leave her..
Broadly speaking Eastern Med is cheaper than Western Med. There are exceptions of course. As there are exceptions in each and every country in the Med. We are currently hauled out in the Ionion Marine yard in Preveza, Greece. Annual price is slightly less than Yet Marine in Turkey where we last hauled for any significant time. There are many yards in Greece that haul on sleds that are several hundred Euro's cheaper than what we are paying. We are 40 feet and cost is 2300/annum plus VAT of 24%. Free water and Elec but that may change if we were aboard for any length of time.
We left our boat on water in Licata, Sicily, this year. 1850 Euro for 9 months + 1 month free for members of RYA and some other cruising/yachting associations. 45' mono. Water and electricity is extra, if you live aboard.