There’s really no straight answer to this question without knowing more about your goals (short and long term) and the parameters surrounding your property. We need to weigh many factors and variables such as the location of the ADU on your property, if the existing infrastructure of your home will support an ADU, your design preferences, how big you want your ADU to be and how many rooms you need, the number of bathrooms, the level of finish materials, size of the kitchen and bathroom, if there’s a laundry room, and more.
Garage conversions may require a lot of work to make them livable even though the shell of the structure exists. Concrete slabs and foundations may need to be removed for power, sewer, and water, and retrofitting of the foundation may be required.
Don’t set your expectations low. Just because it is a small home doesn’t mean it will be cheaper to build; in fact it’s just the opposite due to the economy of scale and reasons mentioned above.
Many companies advertise low costs for an ADU just so you will contact them. Often the cost advertised by other companies only includes the shell of the home (foundation, framing, roof, and windows).
As a general ballpark budget, a detached ADU can cost upwards of $85,000 to design and build. An ADU conversion can cost upwards of $40,000, depending on the current state of the structure to be converted.