£20 in petrol? thats alot, way more then what me and brother would use in a comp. Normaly we would take two bikes full of petrol, £7? then take a full can another £5 then that would last a day, or a comp. The bikes themselves cost about £3500 new, but if you get one seconde hand you could prob get a 2000-2004 about £1500, 2005-2006 £1600-£2200 2007-£2500. Thats a rough price range. Also you won't want anything less than a 125cc. Depending on how heavy/age you are thats what I would go off.