Add porches, decks, patios, interesting trim or railings for design variation to the basic house shape. Avoid complicated roof lines, which are material and labor intensive. Carefully plan, select, and limit amount of windows. Use a bay or Georgian Colonial for visual interest. Windows are an important consideration in any design but they are expensive and can be overdone.
Bathrooms are expensive.
Try having two bathrooms. Instead of a private master bathroom, consider a shared bath with a door into the master bedroom and hallway. Select standard size and average quality cabinets and doors. Consider hollow core six panel doors instead of solid wood doors. Wall and floor coverings can be expensive.