£18 at Fax, though they have different pricing for sitting or standing.
Just had a quick whizz through the websites and got this for the lowest adult price for a single ticket. Obviously, some variation as some grounds are all seater, some aren't.
At £12 - Whitehaven and Hunslet
At £13 - Dewsbury
At £14 - Workington, York, Keighley and, errm, Hunslet
At £15 - Halifax, Sheffield, Batley, Doncaster and Barrow
At £16 - Hunslet again
At £17 - Fev
I got fed up looking for prices for Leigh and Swinton.
"We'll sell you a seat .... but you'll only need the edge of it!"