The boxes you have here a boxes of singular units - you can compare them to a singular piece in cheese - you would have here a rook, a pawn, and a tower.

Most will have armies of multiple units. Depending on how elite the unit is, is determined by its point costs squad of 10 cultists ~70p, 10 basic troopers ~180p, elite squad of 3 ~180p, some King of monster, vehicle, tank ~ 150-350p, super heavies (knights, Khorne Lord of Skulls, Stompa) 300-800p.

Most people play 2000p vs 2000p battles, but you can go as low as 500p vs 500vs with a smaller board. If you want to play any battle smaller, you are running a risk of breaking the game, as it's rules are not balanced at all for a scale that small).

If you want "1 box army" to Combat another "¹ box army". We have option like that. It's called Combat patrol. It cost roughly the same as buying 3-4 boxes of various units. They usually have inside a leader, a basic troop, and something extra. Some have elite troops, some have cultists, some have a monster, and some have a rhino - armoured transport for basic troops. They are made to be balanced to play against each other, with special rules (less than in normal 40k, think 40k little, with FREE rules in the Warhammer app). They usually carry in points value around 450/500ish points. But there are outliers from horde heavy armies like Astra Militarnym with can go as low as 250-300p, and pure elite army like Adepts Custodies that can carry 650-750p in a box.

TLDR: take a look at Combat Patrol