Poor Monty. Good they are doing a culture - I had a foster cat once who my vet tested for resistance and it turned out the antibiotic he was on (ironically, Baytril) was one he was resistant to.
Good you found better pricing - I was staggered at what you originally quoted - these are pricy drugs, but that's way more than I have ever paid for either.
Please keep us posted on your sweet boy, who, as Claudia says, needs to catch a break! Hugs to Monty and to you!