Adjust value of field based on the value of another field

Got it working, but cannot get unit costs not to show till 10 units are obtained. Here is my code:

var v = Number(this.getField("PlatesOrdered").valueAsString);
if (v>=10) {
event.value = "131.99";if (v>19) {
event.value = "128.9";
if (v>49) {
event.value = "124.99";
if (v>99) {
event.value = "122.21";
if (v>249) {
event.value = "118.70"; if (v>449) {
event.value = "116.19"; if (v>999) {
event.value = "113.79";
}}}}}}}

I should add, that I have several other UnitPrice fields that will change values based on the value of PlatesOrdered.