Square of a half-step price

Measure the distance from $100$.

1. Write $99.5=100-0.5$.
2. Then $99.5^2=100^2-2\times 100\times 0.5+0.5^2$.
3. This is $10000-100+0.25$.
4. The result is $9900.25$.

Answer: $9900.25$