how to calculate percentage of chance in a price negotiation I would like to know how to calculate a Percentage of chance in a negotiation. 
Here is the problem : 
The product has a price of 30€ which represents an acceptance rate of 100%
The minimum price accepted is 27€ which represents an acceptance rate of 50%
How to calculate the acceptance % of the prices between 30 and 27 ? 
Thank you for your help.

This is guessing the function $r(p)$, while knowing $r(27€)=50 \%$ and $r(30€)=100 \%$.
If there is nothing else, the simplest thing to do is to assume a linear law, doing linear interpolation
$$
r(p) = r_1 (1 - \lambda) + r_2 \lambda \\
\lambda = \frac{p - p_1}{p_2 - p1}
$$
Example:
$p=28€$, then
$$
r(28€) = 50\% \left( 1 - \frac{28€-27€}{30€-27€}\right) + 100\% \frac{28€-27€}{30€-27€}
= 66.67\%
$$
