At the moment, Manchester Megariders are only £10 on the newish 38 service (Manchester Royal Infirmary to Farnworth via city centre, Salford, Swinton etc). Nothing stopping you from just walking on, buying one and walking straight off again and going and using on the Stagecoach you want to use if it's not that one (they are a standard Megarider, just can only be bought for £10 on that service). £13 on any other Stagecoach bus. Bit of a faff perhaps but maybe worth it for some! They are every 10 minutes daytimes.