You can use pedometers which are less expensive , although less informatoon is obtained.
On the other hand, physical activity questionnaires have error implicit, but are able to classify big groups of population. Thre are specific for diffrente ages.
at some degree, physical effort is measured by % maximum heart rate. any changes in heart rate will indicate physical activity but unfortunetly also stress. heart rate could be measured by watch and fingers so it is most cost effective. here is publication by my mentor.