A while back, I remember one of the developers saying that they crunched the numbers to determine exactly what the value was in buying a new unit, which means that they had a number to use to compare against the value you'd pay for each guaranteed buy. I'd bet that the prices involved in buying a unit directly reflect the probability of getting that unit randomly. The specific buys are NOT intended to be more cost-effective, but rather more time-effective and as such, less likely to irritate people.
If you still have big gaps in your full army and you don't NEED one specific unit to complete your list of doom, then you're probably going to gain more by buying units randomly than by purchasing specific ones.
If you still have big gaps in your full army and you don't NEED one specific unit to complete your list of doom, then you're probably going to gain more by buying units randomly than by purchasing specific ones.