this should be a 8kuy kata.

How is this kata a 6kuy? This is too easy.

Saying that this kata is simple is rather a misnomer. Definitely not a 7kyu.
Might be easy for you if you remember a very specific math lesson from school.
Otherwise there is no chance to deduce the solution.

This thing is definitely not 7kyu.

What was the point of this kata?

wow - this is the most pointless kata - really got annoyed with it.

Just bad kata - why would you put null as param and expect -1;

This is not the best practice, there's a lot of repetition (4 similar .filter fn callbacks), not to mention that the walk array is filtered 4 times.
How do people vote on these solutions?

Not to be snarky, but this kata is completely useless and should be removed.

Why do you use 'self'?

Test cases are wrong:

price: 663000, haggle: medium
663000 - 663000 * 0.3 = 464100

Test fails:
It should work for random inputs too - Expected: '464099.99999999994', instead got: '464100'

What is this?