Hello,
You can build as many mythical units as you like; wether you want to build 20 medusae, 40 pegasi, or 60 manticores - as long as you have sufficient population, you can do so.
As for the price of units; Velerios sells all units for a set ammount of silver, which you can easily calculate yourself with the following calculation: (( [Unit's original wood cost] + [Unit's original rock cost] + [Unit's original silver cost]) * 1.5) = Velerios' price. I've attached an image below to show what I'm refering to:
I hope this answers your question suffciently.
Regards,
Acupuncture
PS: For more info, see http://wiki.en.grepolis.com/wiki/Phoenician_Merchant