Reverse Greek Conversion Golf
Python 3, 112
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lambda x,z=&quot;ΑΒΓΔΕϜΖΗΘΙΚΛΜΝΞΟΠϘΡΣΤΥΦΧΨΩϠ&quot;.find:sum((z(c)%9+1)*10**(z(c)//9)for c in x)

With test cases:
assert(f(&quot;ΡΚΓ&quot;)==123)
assert(f(&quot;Η&quot;)==8)
assert(f(&quot;ΨΟΖ&quot;)==777)
assert(f(&quot;Ρ&quot;)==100)
assert(f(&quot;ΧϜ&quot;)==606)

Loops through the string and uses its index in the list of potentials chars to calculate how much it's worth.