I want to add this up top, here - I started with eleven Runners. That's a bunch of ducks! So what I did required a good bit of space.
Much depends on what kind of shelter they have. I spent hundreds on an outdoor shelter, only to discover my Runners were better off in the walkout basement (it stays much warmer and cooler).
If you have decent space in an existing structure (we can have a long discussion of decent - but it includes no predators, no toxic stuff including fumes, warm enough, cool enough, well ventilated, safe), you can get a number of items they need - a swim pan/kiddie pool, food and water bowls, for free or cheap.
It is important not to skimp on safe outdoor shelter if you will be keeping them outdoors. I kept mine indoors for 3 months because they arrived early and spring was late, and it took me a while to get everything built outside. Again, if I'd known then what I know know, I could have saved myself a few hundred dollars at least.
So the recurring costs are bedding (I use pine shavings, but some folks have access to things like oat straw, flax straw, sawdust) and food. If you want Welsh Harlequins, if they weight 5 or 6 pounds then that's .3 to .5 pounds of food per duck per day. I would set aside some money for a vet visit, just in case. And then there are some home treatment things that are good to have before the emergency strikes.
Epsom salt, triple antibiotic ointment without painkiller, food grade activated charcoal, vet wrap, calcium supplement and a few other items like saline solution (you can make your own, by the way). Those are off the top of my head. There is also poultry vitamins, 4 dollars for a 4 year supply, clear iodine, a few bucks for a small bottle.
It's the initial outlay that can get pricey.
What you can do, also, is use their poo for plant food - so if you are a gardener, that can reduce the cost for fertilizer just a tad. The used bedding makes superb mulch for gardens. So that's another thing you don't need to buy. Or you might be able to trade duck bedding with a gardener for some fresh vegetables. Yum.