Cost price reverts to zero when the quantity is changed

There are several factors that can influence the cost of a jobline.

If you are referring to a material line, there is an option to use Vendor Pricing in the system initialization of Eralis Job. If this is selected, the system will try and determine the cost based on the vendor and the Vendor Pricing as per SAP Business One. In this case, quantity is a factor in terms of the pricing hierarchy.

If you disable the Vendor Pricing option, the system will pick up the cost from the quote cost price list set in system initialization and shouldn't change.

If you are entering other line types, S & R, then quantity is not a factor, but it is determined by the system configuration whether you're getting the cost from the item code or the HR/Resource record.