Floating point numbers are approximations of real numbers, which isn't sufficient in this context. Solving a lossy representation of the problem isn't a reduction in the complexity theoretic sense.
In the case of this problem the set of possible paths is finite so it can, in principle, be sorted in order of increasing cost. The smallest cost delta between any 2 paths in the sorted list is the lowest cost edge in the graph (this probably assumes that all edge costs are positive) so you only need to continue the binary search until intervals of this size are reached. At that point the exact solution has been found.
EDIT: There might be multiple paths with the same minimal cost but binary search will still find this value.
