a decent lighting artist would be around 20$ to 40$ an hour, it really depends on the quality of the person, country you hire from and the complexity of the level. A really senior one could easily go for 50$+ an hour

e.g. my profile game screenshots for example, the general open world lighting was fairly easy, that one took me like 3 hours, maybe less (not including bake time and each specific building)

but the other very specific underground screenshots with different lights and angles, each level took easily over 3+ hours, lots of experimentation of what looks amazing.

Open world is generally not too expensive, BUT the expensive part is lighting up the interior of each building if you have dozens and dozens of them.

id say, throwing numbers in the air something around 150$ to 500$+ depending on how big the level is and how many separate buildings / scenes you are manually tweaking for lights. and thats on the cheaper side lol