Hmmm. In addition to looking for one used (and maybe posting in the WTB thread), in your shoes I might contact G&G, SKB, other case manufacturers, and see what they could do. I can't imagine it would be a whole lot less than $300 though.
MM is now selling Mono cases instead of the old hard cases (not the bag- a padded case). Any chance one of those fits a Mariposa? I'd assume they would have told you when you contacted them, but maybe worth a shot?
Lastly ... and just thinking out loud here ... a "total shot in the dark" approach might be to find people selling a Mariposa on Reverb, eBay, etc. and see if they want to sell the case separately.