The tax collection agency might have an issue with forcing the rounding to the next integer.
Does the calculation work when the sales price is $123,100.00 or $123,200.00?
The county might like the extra breakage but the person paying it might catch it can complain.
I would round the value not use the ceiling or floor.
I would also round the final result since it could be used elsewhere in other calculations.
A possible script:
var salesPrice = +this.getField("salesprice").value;
var transfertax = +this.getField("transfertax").value;
event.value = Number(util.printf("%,1 1.2f", (transfertax / 2) * util.printf("%,1 1.0f", (salesPrice / 500))));