I'm sorry I wish I did know of an inexpensive treatment.
FelV is transmitted through direct contact with an infected animal, so if one cat in your household has this disease than they all are susceptible to it.
Your kitties need to be seen by a Vet and all need to be vaccinated.
I hope you will do this as soon as possible.
To know for sure if it's feline leukemia, you need to have a vet draw blood and do FELV/FIV tests on the cats, as sometimes cats do not show symptoms even though they carry or have the disease. The FELV/FIV test is basically a combination test for both feline leukemia and feline aids. If the cats test negative, then they can be vaccinated but it is wise to test them first.