can any1 tell me cheapest train ticket price from barnsley to london

don't know about from Barnsley..

but Midland Mainline have got a £15 return offer on at the moment, saw it being advertised earlier

http://nrekb.nationalrail.co.uk/promotions/

this should give you some ideas..

 

Midland Mainline Capital Ticket

www.midlandmainline.com

Its £20 if you can get one soon enough. If you're going with others you can also get a 4sight which works out at £12 each for four. Or a 4sight for 2 which is also cheap.

Trains go Sheffield to St Pancras