Actually the cost varies based on your need.
If you just need to the billboard to be seen, and don’t care abou the hot spots, you may just need to buy some flood lights for it. I assume that you need 16pcs (1pc for every 6-8ft gap) of LED Flood lights, the product cost is around $200/pc, so the total product cost would be around $3200, plus the tax and electrician labor cost.
If you want to create an even light distribution on the billboard, you may need to contact some one professional, that’s another story.
BTW, does your company own this billboard? Who will install the ads and lights for you?