To be fair though, the OP isn't asking anyone to choose a house location for him he's just looking for some season ticket price ideas so that he can begin making his own judgments.
Ok.
I have attached something that might help.
LM has the cheapest per mile seasons. The very cheapest on this basis is Shrewsbury -London, which is 222km from Euston (as the crow flies), for £134/wk. This ticket is valid only on ATW & LM trains -a 3 hour journey or more each way.
Apart from LM, the cheapest fares are found on the West of England Main line, with fares very good value the further you travel past Basingstoke.
E.g., Basingstoke - London Via Woking - £98.80/wk, 72km each way (straight line), Tisbury - London -£133.40/wk (144km) each way, Exeter - London (via Honiton) £217.40/wk (255km).
Tisbury is a gruelling 1hr 45 minutes from London, and commuting each day from Exeter on SWT doesn't bear contemplating....
However certainly for instance Andover is a much better value season than Basingstoke, being 30km further, but only £7.50/wk more expensive.
Limiting to seasons under 100km from London, Uckfield is the cheapest, probably because it's at the end of a slow line. It is 64km from London, and £66.70/wk, but a 1hr 15 minute journey.
Looking at some stations with good, fast services to London, around 60 minutes from London, here are some differentials:
Brighton - 78km, 66 minutes to Victoria, £96.50/wk (there is also an £88.30/wk ticket to St Pancras, which is only intended to be valid on FCC services)
Folkestone - London 60 minutes to St Pancras, £144.80/wk (route +HS) ~100km from London
Swindon - 115km, 65 minutes to Paddington, £220.90/wk (this is really obscenely overpriced)
Winchester - 97km, 66 minutes to Waterloo, £113.90/wk
Oxford - 82km, 63 minutes to Paddington, £116/wk
Aylesbury - 57km, 64 minutes to Marylebone, £90.80/wk
Coventry - 136km, 61 minutes to Euston, £219.50/wk (also very expensive)
Kettering - 105km, 62 minutes to St Pancras, £155.50/wk
Peterborough - 117km, 57 minutes to Kings Cross, £172.20/wk
Cambridge - 76km, 60 minutes to Kings Cross, £110/wk
Colchester - 83km, 58 minutes to Liverpool Street, £113.90/wk
Canterbury - London 60 minutes to St Pancras £145.40/wk (route +HS) ~90km from London
The fastest line in terms of distance vs/time is the WCML, but fares here are high. The best value seasons here are on London Midland trains, which basically negates the speed advantage, although the commute time from say Northampton is not too bad, and the speed/distance ratio reasonable.
The most expensive per mile London seasons are the shortest, with Ebbsfleet - London, at £104/wk for 17 minutes/33km into St Pancras top of the heap.
On the other hand, Box Hill - St Pancras (Route Not Underground) is a mere £49.60/wk over the same distance. Caterham - St Pancras at £42.70/wk is also good value.
The most overpriced season of all is arguably Cheltenham Spa - London, at £9,184/yr, 142km from London, a journey taking a painful 2hr 9 minutes.
Templecombe, 2 hr 10 minutes from London, and 169km, is a mere £5,808/yr. Templecombe also has hourly direct trains, whereas from Cheltenham the direct service is only ever two hours, albeit that there are hourly changing options. All-in-all, one can reasonably conclude that FGW rip-off commuters wishing to travel west from Didcot (which has relatively reasonable fares).
For getting the most house for your money, for a reasonable commute time and reasonable season ticket, my bets would be on the MML, WCML, and the further reaches of HS1.