The problem is the one-way. Companies charging ridiculous rates for one-ways is their way of saying "we want that specialty vehicle back where it came from".
As said in the other thread, a broker is the answer. For example, booked from Sixt via BSP Auto you'll get one for less than $70 a day including the one way fee - see below.
It only gets expensive if you need a larger one (as Mercedes and BMW are the last companies offering spacious true four-seat convertibles, for a price) and insist on automatic transmissions.
Autoslash is very good and active on FT: https://www.flyertalk.com/forum/members/autoslash.html, try them as well.