You can use either of them, or both, or none
The wording in this game can be a little confusing, especially on that spell, and they had to improve clarify a few times (source) because it was so confusing.
Thaumaturgic Circles and Sacrifices are two different ways to boost your summoning, as explained under the spell (p.142):
A Domination roll is a contest resolved by an opposed roll: The Magic-User rolls d20 + caster's level + Thaumaturgic Circle Modifiers + Sacrifice Modifiers and the creature rolls d20 +Hit Dice + Number of Powers.
So, your Circle and Sacrifice could be zero (though that is unwise), but they are also unrelated to each other, you could summon with a Circle and no Sacrifices, or with Sacrifices and no Circle.
Each full 2HD of sacrifices gives the caster a +1 bonus to the Domination roll, or 1HD for a +1 bonus if the sacrifice is the same race as the caster. To count as a sacrifice, the victim must be helpless at the time of their slaying and purposefully slain for just this purpose. Combat deaths do not count.
On Thaumaturgic Circles:
The materials used to draw and decorate the circles are crucial to communicating their information to the summoned creatures. 500sp worth of materials is required to invest in a circle for every +1 bonus to the caster's Domination roll.
So, if the interpretation that you combine the bonuses to calculate the circle's cost was correct, you also would also need a similar amount of sacrifices (2 HD per +1 bonus), which clearly wasn't the intent here.
If you check this online summon calculator for the game (aka Summon TPK), you will see that only three things matters for the spell:
- Caster level;
- Desired creature's HD;
- Total bonus to the checks;