Table 2.

Calculation to compare cost per survey of RDS with WebRDS

Cost per recruitment method		Formula	Calculation (AU$)	Total cost (AU$)
Total cost
	RDS	(Number of agencies and locationsa visited*mean time spent at each location*number of study staff*cost to pay 1 study staff per hour)+number of agencies and locationsa visited*number of study staff*(mean distance traveled to location*cost per km of travel)+number participants recruited by RDS*administration time per participant*cost to pay 1 study staff per hour	(45*3*2*30)+(45*2*30*.65)+(232*0.5*30)	13,335
	WebRDS	Cost of Facebook campaign+(hours of work by 1 staff member*number of staff*cost to pay 1 study staff per hour)+number participants recruited by WebRDS*administration time per participant*cost to pay 1 study staff per hour	430.44+((38*8)*30)+780*0.5*30	21,350.44
Per participant
	RDS	Total cost of RDS/number of participants (seeds and referrals) recruited by RDS	12,390/232	53.41
	WebRDS	Total cost of WebRDS/number of participants (seeds and referrals) recruited by WebRDS	21,250.44/780	27.24

^a Number of agencies and locations visited were n=25 and n=20, respectively.