I assume you are travelling for the football.
If you are happy using Advances (i.e. valid only at train(s) specified when booking), then you could do it for £35.30 each way by leaving at 0736 and returning at 1724, if you can be sure of being back at Stevenage station that quickly.
As for splitting, a split at Doncaster makes sense as it maximises the use of day returns. But splitting at Grantham might not be wise: if you miss the 1724, the next (and last) connecting train from Stevenage is at 1829 and doesn't call at Grantham.