Cheapest fares will be Advance singles each way. TPE have released fares up until the 15th, so you'll have to wait to book the return journey, but at present you can get the Manchester-Edinburgh leg for £44.20 for all of you on the direct 1216 service (arrives 1539).
I'd expect if you keep an eye out you could get tickets for a similar price for the return, though as it's a Sunday the fares may be higher (I'm not familiar with TPE's practices so I can't comment for sure).
Bear in mind the Off Peak Return (route:via York) comes to £160.60 total, so you're saving £70 (if you can get the return leg for the same price as the outbound) at the expense of having no flexibility.
Finally, I think that there's some benefit to booking through TPE's website but can't recall exactly what it is. (edit: seems to be 50% off for 16-25 holders; so no use here).
