not affordable, as BD need to upgrade other sector though which is had a more even dire needs (like fighter, beginner trainer aircraft, transport aircraft) and given a chance for a much more lower cost MPA aircraft like CN-235 or ATR-72 series. Remember, BD only had 2 to 3 billion US dollar to be spend for the entire branch of armed forces, and most of them had their own programme too.
Current BD defence spending runs at 3.5 US billion dollars a year, when you take into account arms imports which are not part of the official defence budget.
Who says that BD needs to buy all aircraft in one go? It could make a purchase of 12 and then deliveries could be stretched out over 6 years with the first not starting till 2018. BD will only need to pay as the aircraft are delivered.
You forget that BD economy and defence budget is growing at 7% a year. Just this year, the government hiked defence spending by a massive 20% - ok some of this was for increased salaries but you can see the intention here.