It is difficult but its not because of the price, although that's there too. It's that developing countries may not have the infrastructure to transport and store the vaccine at the recommended temperature of -70C.
The moderna vaccine does not need -70c, juste -20 to be stored indefinitely (regular freezer), and even higher than that if you can ship relatively quickly (i.e weeks)
Developed countries don't have infrastructure for -70C distribution. The US is going to struggle to actually distribute the Pfizer vaccine. It's not like your local CVS has a freezer that goes down to -70C. Dry ice is possible but there's a shortage of that, too. Nobody has ever distributed a vaccine that needs to be stored at -70C.
The Moderna one is much easier, you just need a normal freezer. Which is fine for developed countries, will still be a challenge elsewhere (but one they're used to, it's not the first vaccine that needs to be kept cold)
There is no shortage of dry ice. The input (CO2) is a byproduct of industrial processes (ethanol in particular) and so the market is completely glutted. It is also used for a number of industrial process so there is plenty of infrastructure to produce it in any city.
