I think the guns are pretty expensive on the whole, but most Iphone games are like that, they all have expensive items they'll take you days of farming to get or weeks / months.
Increasing HP would make it much better to compete with the higher damage guns, being able to take more hits than dying in 6.
Bases are pretty small so it's hard to even compete with high damage guns, but why not use grenades, tactical shields and actually work as a team to get past?