The trick is using the vet. How much was a het for albino boa, and how much will you get for your sunglow babies? Your vet bills will pale in comparison.
Money aside, there is much more thought given to presciptions than knowing the dose and method of administration. The veterinarian takes into consideration how the medication will be absorbed, distributed, metabolised and then excreted. This is complicated by a sick boa that is not functioning properly. A perfect example is that the aforementioned amakacin is toxic to the kidneys and will cause permenent injury. This is avoided in otherwise healthy animals because the kidneys can excrete it before harm. The kidneys slow down in dehydrated, ill snakes, allowing toxic changes to occur.
Diagnostics are very important. A cheap and simple culture and sensitivity will find out what is causing the problem (besides the initial inadequate husbandry). It will also tell you what antibiotics will best kill the pathogens if initial treatment fails. If your boa is as sick as you say, bloodwork will be important as well. This will give you an indication of how the immune system is reacting. This information will be important with the drug choice, length of treatment, and if other theraputics are necessary.
Xrays will tell you if pneumonia is likely; again affecting the treatment and prognosis.
Good luck with your new snake and I hope the information I"ve provided will help you out 
Ian
