Question

i guess it is gone downhill . i will leave 15 percent . i could leave less but i don't want to be a cheapskate.

Answer 1

حدس می زنم که (الآن) قیمتش افت کرده. / وضع / بازار اون بدتر شده. / رو به نزوله. / رو به سقوطه.

go downhill
if a situation goes downhill, it gets worse
= deteriorate
Grandma fell and broke her leg, and she went downhill quite rapidly after that.