Re-using soil is a bad idea unless you really have lots of experience.
1) Used soil has lots of salts built up in it.
2) Important trace elements have been depleted.

So unless you really have a green thumb & know what the soil needs, the cheapest and easiest way is just start with new.
As for me I put out considerable energy/money into a grow & I consider reused soil a variable I won't have to deal with.
But thats me.
Peace