Someone must have sat down and figured it out.

Are you better off purchasing a 5 star weapon at whatever price, or a 4 star weapon for $20 less? Or a 3 star for even less, and then upgrading?

While I'll wait to build up credits, I wont wait 12 years to build up 2,000 a-coins for free, so I'll be buying the star upgrade with discounted a-coins. My sense is that it's probably similar, just one way takes longer. 

Also, does it make any sense to hang a 3 or 4 star weapon on a 5 star mech?