In general type of weld, welding requirement, works complexity degree & labour cost.
Type of weld - TIG / MIG etc
Welding requirement - Length, width, depth & way of weld. (some customers have their own standard way of welding which is exclusive.
Work Complexity - Let say a structure has 30 points from different direction hence you may have to make few turns of structure rotation and resulting longer time needed to complete the works and risks involved when handling those heavy stuff. Normally it should charge more than fix point rate like 1.3 x FIXED RATE
Labour cost - It is very subjective as skilled worker come in few grades by experience. Hence you have to determine yourself what work it suitable for specify worker. The hour rate can be divided by estimate welding points can be completed within an hour.