Odoo has given two rules. 

1. pick only one price 

2. pick the most cheapest price 

where these two conditions will be fulfilled that price will be final. 

so we will apply the price in 3 situations and pick the cheapest option. 

1.  1 day =50$,   8 days = 50$*8=400$

2. 3 days=110$, 9 days= 110*3= 330$ (here we can't break into 8 days so we have to pick multiple only)

3. 1 week= 200$ 2weeks= 200*2=400$ (again we can't separate days from weeks and have to pick multiple in which that time period falls. 

so the cheapest option is 2. 

Actually the computation method does not allow to mix lines, this means price will be computed with 3x   3days= 110$ = 330$ (This is the cheapest to cover the time period using one line).