Assuming that a "dose" has a defined time-span. It is not indicated that you need to keep taking doses over the 60 day period so does not necessarily follow that you can take a smaller dose with a reduced duration. It doesn't actually say it lasts 60 days, we tend to assume it is 60 days as that is what the time compression is expressed in, and that may be an absolute. In CT the description said a dose lasts 60 days, if you wanted a shorter duration you needed the antidote (at an eyewatering Cr900).
In reality when would you take fast and want it to ever last 60 days. MGT2 talks about it being an alternative to a Low Berth, a dose would be optimised for that, in a general sense you want it to last until you don't need it. If stranded you might want it to last more than 2 months, if using it for travel you only want it to last while the boring bit is happening. It may simply be an on-off type drug, when you take it you operate at the decreased metabolism until you take the antidote, and is "one dose" regardless of the time interval.
There is no indication that the cost of the drug is pro-rata either. Even if you could make it last a variable time, there is no reason why it would necessarily be cheaper for a shorter time period. For most medication if you want a longer effect you take several doses over a period of time, not a larger dose immediately since that will likely kill you. If you take quarter of a dose it may still last 60 days, but have quarter (or less) of the effect (only slowing time to a 15th). That would mean a subjective journey of 24 hours which makes the logistics a bit more difficult and still require the antidote.
If it were cheaper than a low berth, then no-one would use Low berths. Since Low berths and Fast Drug have always co-existed I tend to assume that Fast Drug is not preferred for some reason. Likely it is the relative profit margins and the convenience to the operator. For Fast to win the dose needs to be less than Cr50 per jump to equal the Low Berth, the delta in equipment costs are marginal so I can't see that being a telling factor. You need to to charge Cr500 plus your costs for it to be more profitable than carrying freight (which have no costs over running empty).
MGT2 says that Fast Drug is used "as a cheap substitute for a cryoberth". On face value that implies that travelling on it should be cheaper than going Low, but it doesn't actually say that. Even if Fast Drug did cost Cr500 (drug, antidote plus some life support) per jump and a low berth cost Cr50 per jump, it would take a lot of jumps for the cost of travelling on Fast Drug to be more expensive than the capital cost of the cryoberth plus the running cost.
It depends entirely on your table of course.
