Amazon offers Prime members the Empire Level 24" True Blue Magnetic Level, model no. EM71.24, for $23.48 with free shipping. (Non-Prime members can spend $35 or more to qualify for free shipping; otherwise, shipping adds $6.45.) That's the lowest total price we could find by $20. It purports to be accurate to within .00050" per inch and features strong rare earth magnets.
Update: Note that this item now has an estimated shipping time of one to three months.