The minimal value allotted to a more probable horse is

 --                --  
|  N  -   0.5 N K    |
|       ----------   |  q
|       n min{q }    |
|              i     |
 --                --
where K is the number of probability coordinates less than 1n, and q is the smallest probability greater than 1n. We know that
 
         |  1-1/n   |
   K <   | -------- |
         |  min{q } |
         |       i  |
          --      --

The minimal winning amount when a more probable horse wins, is hence

 --               --  
|  N  -   0.5 NK    |
|       ----------  |  nq
|       n min{q }   |
|              i    |
 --               --

Solve for this quantity greater than or equal to N.