It's deceptive the way they do that so I'd never fall for it. Best to wait and wait and get the premium stuff for a couple of dollars once they've rung all the cash out they can. 

I noticed today they've dropped the bonus amount of a coins in the premium section.

What a great question to ask! 

Usually these combo offers are best for the first tier reward (1.99 for a 2 star Bastion is not bad); however, some players have the offer available at higher rate (7.99 for 3 star Bastion). As with every purchase, this depends largely on how much $$ you have and what level you are playing. If your hangar is below 5k SP , a $2 deal for a legendary mech is really good, for instance. If you are higher level player, it might be better to wait and get the mech at higher stars to max out faster.

As with every player, we all want to make good use of our resources (in-game and also IRL). These combo offers are a good way to get a mech you do not have as long as you are willing to grind (open crates) over a few months to build them up gradually. I personally choose to do it this way so that I can enjoy the mech at every tier and also work on achievements with various weapons.