If I have only one infected nail, can I treat only that one and save time and cost?

Unfortunately, no.  The reason for this is that if one of your nails is infected,  the chances are that your other nails are infected as well. To allow the treatment to be successful and prevent future self infections, it is best to treat all the nails at once.  An exception to this is for the treatment of an isolated fungal infection related to acrylic nail air pockets. In these incidences, we will treat the one affected finger nail.