Pharma is about the only sector of the health industry that can set its own prices. The fact the the USA does not have a national healthcare system is the major reason why drugs are hugely more expensive in the States. The NHS (or the EU, in some cases) can negotiate volume discounts whereas the USA system invariably involves individual insurance companies or often, individual patients ,trying to negotiate on their own. The Republicans have always fought against a centralised negotiating system and have, in effect, allowed Pharma companies to set their own prices with no regard for anything. The advantage of our system is that NICE (National Institue for Clinical Excellence) agrees the 'value' of particular interventions and drugs. If that is too high, it will not approve them and the Pharma companies know this. What the NHS gives them though is some certainty. If NHS England says it will be funded (at a certain price or lower) then that is guaranteed. In the US, some insurance companies might say yes and others will say no. For any business, that is a difficult square to circle.
Admin costs when dealing with multiple companies are also much higher than when dealing with a non-National system.
For the ever increasing 'speciality drugs', which service a very small population, the price differences are even more marked. 20 year patents also, effectively, secure a monopoly position for many Pharma companies and, even the most die hard capitalist will admit that is far from an ideal situation.
All that is before you get to Orphan drugs.
Many Americans do not fill their prescriptions or take half doses to make them last longer, simply due to the cost, which cannot be a beneficial outcome.
Ultimately though, if you are purely profit focused, assuming similar production costs, would you rather make 1 pill and sell it for £1M or make a thousand pills and sell them for £1500?
All this suggests that Mr Trump has assumed (wrongly) that the USA is subsidising the rest of the world and this needs to stop. The truth is that the USA has a higher cost due to limited supply and a model which inherently promotes access for the few or the '1 pill for £1M' model over everything else. What the danger to the NHS is, is that a similar model is adopted here.