Where did you get $750.00? The only problem with MyCryptoCheckout is the $59.00 yearly fee for unlimited transactions while the free version you only get 5 transactions.
I was thinking a specific FIRO only payment gateway plugin, just like the Monero one. While the dev could charge a fee per installation, cutting down on initial development costs.
I would definitely donate to this development or initially purchase about 10 licenses for the plugin.