That's a good approach, but I'm not sure anyone can really tell you how much money to have on hand unless they have some way of knowing what the problem is going to be. There are routine things like Fecal Exams and such that run in the neighborhood of $35.00 for the office visit (if required) and $25.00 for the test, depending on where you are. I do my own Fecals, so that saves me quite a bit of money because it's something that you'll want to do probably anually or so as a preventive measure. Some of us on here have a bunch of them, so that can get pricey.
If you end up in a situation where emergency surgery is required, it can run from a couple hundred dollars to several hundred dollars (or even in the thousands) depending on the problem requiring the surgery, follow-up care, etc.
Medications are normally not too bad, usually it's just going to be a wormer or something like that or possibly Baytril for an RI.
The bottom line is the better your husbandry, the less chance you'll need the vet. The exception to that rule is when you get into breeding them and run into unpreventables like egg-binding. There probably are ways to prevent that too, but we aren't exactly sure what they are yet. Successfully and consistently breeding Drymarchon is still a science in it's infancy. We certainly have more questions than answers still.
There is a wealth of in formation on husbandry and breeding (at least what is known so far) on this forum and in the archives. All you need to do is use the search function and you'll probably find at least one discussion on just about any subject pertaining to Drys. If you don't find what you are looking for, then jump on here and ask. Someone will have an answer for you.