I agree that the films budget needs to likely be in that $75m range +/-, absolutely no more than $100m at the most.
The dynamic that by including Pym, and another Ant-Man, is that in an Ant-Man sequel or Avengers film you could then have Pym assume his other most famous identity of Giant Man
(I like Goliath better as a code name) while Lang or O'Grady is still Ant-Man.
Giant Man could come in handy in a situation where Thanos is at play and as much muscle as possible, maybe two fronts, is needed. Hulk and Thor busy on one front with Giant Man at another.
Pym also ends up creating one of the most deadliest pieces of AI out there named Ultron
. An ever evolving sentient robot whose first brain patterns were that of Pym, making him at his core a twisted maniacal Pym in some regards. Ultron goes up against the Avengers a number of times.
So having Pym allows for these avenues to be opened up within the MCU.