Device prices have little to do with it.
Context: The average Burmese citizen earns about $5/day
Network SIMs were (and are) heavily government controlled and used to be upwards of US$1000 apiece to buy, plus operating charges.
New, very cheap (about $2) SIMs were introduced at the start of the year but these are for the CDMA network operated by the Army, are extremely limited in supply (ballots among villages are usually oversubscribed by more than 20:1) and of course won't work in GSM phones.
Myanmar/Burma has competing GSM and CDMA networks.
GSM SIM prices have come down a lot since the start of the year but they are also in very short supply. You can wait 6-8 months for one at the official $1.50 rate or buy on the black market at the prevailing rate of about $80
Phones are getting a lot cheaper (they used to be extremely heavily taxed at the border) but the real barrier to entry has been high startup/operating costs and the reduction in those prices is what's driving sales at the moment.
Things will change further over the next year as several companies are rolling out new GSM networks. I'm aware of 2 companies actively recruiting people with turnkey installation experience right now and another 2 seeking staff for the management side.
@Stilted Banker: The Burma/Myanmar name issue is complex. You will find that both names are used inside the country and a lot of people do not like the name change, feeling the military oligarchy had no right to impose it on the people without consultation. Others just want to move forward and make up for 60 years of stagnation. Given there are still 3 civil wars going on in different parts of the country it's hard to run a referendum and get consensus.