Please help us to help you by providing the dates of your travel (and any restrictions on times you might have). Also tell us where in Cheshire you're travelling from.
For example, if you were travelling out on Friday 4 December from Chester, the single fare to St Ives is £40 on the 0618 service, getting you to St Ives at 1531 (with changes at Newport, Exeter & St Erth). Returning on Sunday 6 December costs £63.60 on several services, such as the 1503 which gets you back to Chester at 0022. (Change at St Erth and then at St Austell for a rail replacement bus to Plymouth. Then change again at Birmingham). So that gets your return fare down to £103.60.
The ONLY place you should be looking for rail fares is here:
http://www.nationalrail.co.uk/
Third-party ticket outlets can't offer you anything cheaper. Further, they often add on unnecessary booking fees and they've even been known to issue tickets that aren't actually valid for travel. (I used to run a railway station and I saw quite a few of them!).
Once you've provided your exact requirements, AB's three rail travel experts (Dzug2, Mushroom25 & Buenchico) can get to work to see if there any cheap options for you.