The cost of building a custom laundry room can vary based on a number of factors, such as; the size of the room; the specifications of the appliances; bespoke fittings; additional plumbing and electrical requirements; and the quality of the finishings.
A small and simple laundry room with no changes to the plumbing, electrics, or room layout can be achieved for as little as $2k-$3k. However, a large, custom-designed laundry room built to a high specification can cost up to $15k.
In most cases, you can expect a laundry room to cost somewhere in the middle of these two figures but luckily, this is entirely dependent on you and your laundry room design so there is a workable solution for any budget.