On the rear wheel, it means brazing new tabs. Probably not overly expensive, especially if you have connections. It's the repainting that costs a lot, so think about it only if your bike is due for repainting. In the front, you need a new beefier fork. Will you be able to match the colour? That being said, there are drawbacks to such a conversion:

– The typical rear brake (installed on top of the chainstay) limits severely your selection of rear racks. So a proper rack costs more.

– In the front, a disc-brake-compatible fork will be beefier, therefore will have less shock-absorbing capacities.