When my cat stopped eating and drinking a few months ago the only thing he would eat was yoghurt. Two vet visits in less than two weeks and a battery of tests was most definitely a financial hardship for me but losing my beloved cat would have been much more devastating. Please, please, if there is any way possible for you to get the necessary tests done, do it now. ASAP. As Maya said, talk to the vet about a possible payment plan. You never know unless you ask.