Im not saying theres no limit like it’s obvious with gods and it does do things sometimes but there are other times its ignored and servants have abilities/NPs higher than what youd think should be possible within a class container.
Spirit Origin and Spiritual Core are two different things.
Spiritual Cores - When a Heroic Spirit emerges into this world, he first acquires a Spiritual Core. The Heroic Spirit then materialises as the body envelopes this Spiritual Core. In order to defeat a Heroic Spirit, one must inflict damage to the Spiritual Core. The Spiritual Core gradually diminishes as a result of massive magical energy expenditure or sustaining bodily damage. Under these adverse conditions, the Heroic Spirit’s expenditure of magical energy will escalate, and should damaged be inflicted through powerful magical energy, curse, or Noble Phantasm, the Spiritual Core will be destroyed, and the Heroic Spirit will no long be able to remain materialised. The heart and head are connected directly to the Spiritual Core, thus they are the weaknesses of Heroic Spirits. Sustaining damage at these locations will significantly weaken the Spiritual Core.
Spirit Origin - Saint Graph, basically a temporary soul, that has the personality and memories, and does the ''decision making''. The quality of the Saint Graph defines the limits of the Servant, and is affected by Master's potential especially as far as Magical Energy capacity goes. Basically, the Spiritual Core is the ''thing that has form'' and the Spirit Origin is the ''power'' that fills that form.
The Class Container, by itself, isn't cheap in any way. The Spirit Origin that fills said Class is the ''cheap'' part, at least when compared to what it could be (Grand)
Heroic Spirits by themselves are only power. They lack the concept of a body/form that's necessary for them to be summoned as Servants. The Class is that ''FORM'' in very broad terms. Now, if the form is filled with dirt or filled with iron, to use ''examples'', that's another story.