For existing MidiBridge customers, there is now a permanent discount (currently 60%) off the price of MidiFire via an App Store bundle.
Follow this link to visit bundle page on the App Store.
It works both ways: new MidiFire customers who might like Akai SynthStation25/Line6 Midi Mobilizer I support (ios11 and below only) can pick up MidiBridge at a discount too.
Note, you need to be logged into the App Store recently to see the 'complete my bundle price', although if you do buy the bundle before logging in Apple will realise and only charge you the difference between what you paid for MidiBridge and the bundle price.
Important! - In order to take advantage of the crossgrade discount you must purchase the bundle. Do not purchase MidiFire from its own App Store page (you will not get any discount) - always use the bundle page (link above) if you already own MidiBridge.