Lighting is the cheapest way to change how a room looks and the easiest line to overspend on, because the difference between a transformed room and an expensive one is entirely about which fixtures you hire. It is priced by the number of units, the length of the run and the labour to install and collect, and almost all of it is hired rather than bought.
The three things people actually buy
Uplighting is a set of fixtures placed around the walls that washes the room in colour, and it does more for photographs than for atmosphere. Festoon or fairy lighting is a run of lamps overhead, priced by the metre plus the rigging, and it is what people mean when they say a marquee looked magical. Dance floor lighting is a set of moving or static effects over the floor and is normally part of a DJ's own kit rather than a separate hire. Knowing which of the three you want makes quotes comparable instantly, because suppliers price them in completely different units and a single number for lighting hides which you are getting.
What moves the price
Unit count moves uplighting. Run length and rigging complexity move festoons, and rigging is where the labour is: lights strung from a marquee frame are straightforward and lights strung across a listed room with no fixing points are not. Access and time move everything, because a venue that only allows setup on the morning of the wedding forces a compressed install with more people. Power is the constraint everybody forgets: a field with a generator has a fixed capacity shared with the caterer, the band and the bar, and a lighting plan that ignores it will trip the site.
Where it is worth the money
In a plain room, lighting is the highest-return line on the whole budget, because it changes every photograph taken after dark for a fraction of what any other change would cost. In a room that already has character, particularly one with good architectural lighting of its own, it adds much less and sometimes fights what is there. Ask the venue what they already have and what other couples have hired, and ask the supplier to see photographs of your actual room lit rather than a gallery of someone else's. Both questions cost nothing and they decide whether this line is a bargain or a waste.
Questions people ask about event lighting
How is lighting priced?
Uplighting by the number of fixtures, festoons by the metre plus rigging, and everything by the labour to install and collect. Ask which units a quote is counting before comparing two of them.
Is uplighting worth it?
In a plain room, yes, and it shows in every photograph taken after dark. In a room with strong lighting of its own it adds much less, so ask to see your room lit rather than a portfolio.
Can the DJ do the lighting?
For the dance floor, usually. For room lighting, sometimes, and it is often cheaper as part of the same booking. Ask, because it saves a supplier and a delivery.
What about power at an outdoor wedding?
It is the binding constraint. Ask the generator supplier for the capacity and give it to the lighting company, the caterer and the band, because they are all drawing from the same source.