This may sound foolish, but dig a couple small test holes in your dirt floor to verify there is nothing underneath. Dirt in second picture appears to be level with top of sill plate.
Simply using a plate compactor on your dirt floor and filling in low spots would be my suggestion if you don’t need a concrete floor.
Ag lime and crusher fines are typically very cheap. However, depending on your proximity trucking may not be and the labor to spread and compact may leave you feeling $3,000 wasn’t such a terrible investment. Also, there is a difference between ag lime and limestone crusher fines even though they are both limestone products. Crusher fines are more widely available - both would work just fine.